Hero Guide to Spectre:
Spectre is in my opinion the most versatile hero I ever had the pleasure of playing. He has so many different playing styles and roles, but most times you only see the same old cliche builds that works in pub games. He can be a global controller, tank, dps, assassin, flank, and more. This is actually rare for an agil hero as most of them are simply DPS.
Heart + Radiance:
This is the most common build that people talk about and they're going to be mad when I say this is perhaps the stupidest build ever. It only works in pub games, I guarantee it.
Here's why.Radiance: Rushing a radiance is a huge gamble. A good radiance can be rushed under 15 minutes, but the problem is, what are you going to do once you have radiance and with only boots? You still cannot contribute for several minutes. The most you can do is ult and use radiance to scare off a few low hp light agil/int heroes during a team fight. A weak hero running away has a good chance of TP away before you can kill him. But any decent player when seeing a radiance spec, will simply target and feed off him for the first 30 minutes b/c he knows that Spectre is literally food. In fact, I just recently watched Letjing do that to a poor Spectre with his Slardar.
To be blunt, radiance is a dumb item for Spectre considering the cost and what it does in comparison to other items he can get or need. The only time you would need it is to counter image heroes or you are going global build, which I will get to later.
Heart: Usually if a radiance Spectre is able to get it by 15 and survives enough, the 2nd item he would get is a heart. Heart is a dumb item generally, again considering the cost and what it does. This is a well known thing back in the days even in pub games, I'm so surprised this information is lost in modern games. All heart does is give you bulk, which is easily countered by DPS. It would make sense for some STR heroes to get it b/c the STR gain will also give you damage, but for ANY tank, you need both bulk and armor to be successful. With just heart, DPS will cut him like butter. Spectre is an agil hero, even with heart he is cheese. His dispersion passive helps, but compare to other items you can get, such as satanic or skadi, it is completely ineffective. In fact, I pretty much never get heart for any hero considering the other items you can get.
Suggested DPS Build: Focus on Desolation
As a DPS build, the suggested bulk item is SKADI, always! Skadi will give you bulk, the stats gives you damage, armor, mana, and IAS. AND it gives you frost, which for a DPS melee is important. Never go SnY, it's a level 2 item that helps early, but meaningless late. Either way, skadi is superior choice than Heart in every regard. Think about you, you are fighting PA, that frost lowers PA's IAS while a heart does nothing.
Now if you need to take on LC or some big heroes, you might need a 2nd bulk item which should be satanic, not heart. Satanic does stack with skadi in this current version, making it a very good combo of frost and lifesteal. Now a Spectre with skadi and satanic is more than enough to sustain any fights even taking on tanks.
If you do not need the extra bulk of Satanic, then diffusal is the recommended item to stack skadi. The reason diffusal is core for any image heroes is because your images all get the same manaburn ability. If you are planning on diffusal, get it before skadi as it is cheaper and more offensive.
From this point as a DPS, you should get a Abyssal Blade as bash with fast IAS items are always key to take down big heroes.
These are the core items for a DPS Spectre, with the rest depending on the game. A manta is great for this hero for obvious reasons, especially if you want a little more movement speed (which he does need despite having dagger). Manta always > Sny, especially when you are getting a skadi already. Butterfly and Cuirass are also good b/c they help you tank more and gives you IAS for that bash. Cuirass reduces armor of your enemy which is more offensive, while butterfly's evasion helps you survive vs bashers.
These item combos alone makes you a headache for your enemies. And that is only about 5 items, leaving the 6th item to whatever you want. This hero is so OP it doesn't really matter at this point. Get a crit, or get a guinsoo, or get an orchid.
Items: STR Tread/BoT, Skadi, diff/satanic/mjolnir (for image heroes), Abyssal, butter/cuirass, manta. That's 5 items. Last item - anything you want, even radiance if they have image heroes.
Skills: Dagger, Dispersion, Desolate, Desolate, Desolate, Ult, Desolate, then dagger all the way if you want to be more offensive or dispersion if you want to be more tanky to farm.
Suggested Tank Build: Focus on Dispersion
If you're going to tank, basically only the item orders differ as you want to incorporate a BM early. Also, there are 2 base items you want to consider. One is Mjolnir, if you want to be more of an aggressive tank and also if they have image heroes. 2nd is satanic if you want to be more tankier tank. And both stacks with skadi. DO NOT GET HEART, unless you plan on standing there doing nothing while they hit you. Of course, you need BM. Some might get VG to start, but I'd skip it and go directly to skadi after BM.
Mjolnir build: STR Tread, RoA, poison orb, point booster, BM, then start saving for mjolnir, then finish skadi, then cuirass/butterfly, then whatever else you want to tank. Final items: Tread/BoT, BM, Mjolnir, Skadi, Cuirass/Butterfly, and last item.... whatever you want, more tank? Radiance? Abyssal?
Satanic build: STR Tread, RoA, poison orb, pt booster, BM, finish skadi, then satanic, then cuirass/butterfly, etc... Final items: Tread/BoT, BM, Satanic, Skadi, Cuirass/Butterfly, and last item.... whatever you want, more tank? Radiance? Abyssal?
Skills: Dagger, Dispersion, Dispersion, Dagger, Dispersion, Ult, Dispersion, then dagger all the way before desolate. In extreme cases, skip desolate all together since you are tanking and +2 is better.
Global Controller: Focus on Haunt
I love this build, I absolutely love this build.
You choose items which build on Haunt's images. First is diffusal, always, doing massive damage with your ult and at an affordable price. Use it to chase and take down weak heroes too. 2nd is radiance if game is in your favor, but skadi is most recommended. Why? Your images gets some of the frost (at least it use to not sure if it still does). Either way, you get the slow. Slow allows your images to attack more as they get away. 4th item is cuirass. This is the one time cuirass > butterfly. Cuirass cuts armor and your images get the aura (again at least it use to, I haven't checked recently on the newest version). This increases the damage they take from your ult. Also the IAS allows more hits per image. Last item... refresher. Oh yeah, baby. 
Don't cast your refresher simultaneously with your ult. People tend to run when you ult, but afterward, they let their guards down. Use it sequentially. Ult first, and with your combined items above, will severely weakened any agil/int support even at lategame. Then when their guard is down and your team engaged, you ult a 2nd time. Wait a few seconds before tp in and finish them up. They won't know what hit them.
Now this build is obviously not as effective in battling biggies such as lategame LC etc... It is a global controller build, so make sure your team has a lategame bigboy like Void or Skeleton King or Naix, etc... if you are facing big LC or Slark, etc...
Items: Tread/BoT, Dif, Radiance, Skadi, Cuirass, Refresher
Skills: Same as DPS build.
Assassin: Focus on Haunt
This build I've talked about to death in the forums and many people are against it b/c it is unorthodox.
You can rush a dagon in 10-12 minutes. Unlike a radiance, a dagon with 400 blast can get you instant kills on heroes running away or in team fights, you strategically focus on their weak back lineups of int and agils. This is the same strategy used on invisible heroes going dagon or SB/Furion going dagon. Except it works better.
The problem with a lot of complainers is they assume you are going dagon 5, which is not the case. You get the dagon early, it supplement your manapool and it only cost you 10-12 minutes, which you easily farm back with the dagon. It is an early step to control late heroes by denying them early food or deny them ganks, when you can instantly pop into a fight and blast 400. Stacking with his dagger's damage, you can see why it is extremely hard for low hp heroes to run away even with TP.
It works better on Spectre for various reasons. One is Spectre's dispersion makes him tougher naturally than say BH, SA or Furion with an early dagon. 2nd, Spectre is global and pops up instantly next to the enemy. I've taken down 1400-1600 elo players with this build, frustrate them after 10 minutes. Those players rely on their supports to carry them the first 20+ minutes while they farm their big items. They have idea how to counter a Spectre that hunts them this early. If they counter with items, then they're sacrificing their core items for lategame, it delays their growth.
You use your ult, tp in and take a kill. In between your ult, use the dagon like hands of midas and farm big creeps. The dagon is only for clean up while you continue to farm your build for the other roles such as above - DPS, TANK, GLOBAL, etc... If you already have tanks on your team, you don't want to go tank with Spectre. If you already have some DPS, you probably don't need another DPS Spectre. If you are lacking early game heroes to control the first 20-30 minutes, you might need another nuker.
Anyways, an effective 2nd item for this hero is Book, but again, only if you feel comfortable and you don't need another lategame DPS or tank. Book is one of the most underrated items in the game, the level 3 book necros do massive damage, manaburn, and when they die, they distribute heavy damage. I've games where I tower rush with my ult to kill a run away enemy but another enemy was waiting. I got the kill but couldn't run away before I got killed, but then while dead, I attack the remaining hero with the necros. They underestimate the necros thinking they got free gold at the tower, wrong and I got a 2nd kill.
Either way, going book or not, after this you still need to choose DPS or TANK or something and get items you need. Radiance in this case might be good, along with Shiva. Your job is to pop up behind enemy lines and take out their silencer, lina, Sniper, etc.. and quickly. While lategame you can still virtually go any build you want. This hero is so damn OP, that's why he is a noob hero.
Items: Tread/BoT, Dagon level 1 (sell it later), Book (if you like especially with invisibles), SKADI (as this item is necessary for ALL Spectre's build), then whatever you like!
Skills: Dagger, Dispersion, Dagger, Dispersion, Dagger, Ult, Dagger, Dispersion, Desolate, etc....
* NOTE: I should mentioned that I think often people take me too seriously. While I standby the assassin build and the reasoning behind it, it is not the motivation on "why" I play this build. I play for fun, for enjoyment. So I wouldn't play a build just because it is most efficient or necessary at times. I play a lot of Dagon Spec for 1 reason and 1 reason alone, it is hilarious.
Dagon Spec does carry 1 very real risk of gamble. If you cannot get the kills early to gain the money spent on the dagon, you essentially fall behind and waste the money. So it is recommended only when they have a lot of squishy heroes or you use it wisely to clean up. You must get kills for each ult and hopefully you get to ult as often as it CD. That way the kills will pay for the dagon itself, else you find yourself in mid game struggling for items.
Conclusion:
In the pub game these days, you can get away with heart and radiance on Spectre if you like. In most games you'll probably own. But if you play against decent players, players who know fundamentals, know how to counter a heart and radiance, you'll find most Spectres can't even reach 30 minutes let alone 40 minutes and then find their builds just completely countered. Just because this is a "noob hero" it doesn't mean you can't actually learn the fundamentals of this hero and make it a real threat to actual pro players out there. Dota is all about learning how to counter and recounter and counter counter.
I should actually write a guide on how to counter Spectre to force other Spectre players to rethink their Spectre strategies. But anyhow, good luck and have fun!