It's funny how the spy class in TF2 reminds me of the rogues in World of Warcraft. The spy's special skill, cloaking, seems alot like the rogue's vanish skill. However, one aspect in which the spy feels and plays more cooler than its WoW counterpart is the fact that he can disguise himself as the enemy, allowing him to infiltrate into their ranks. On paper this sounds very easy, but be assured that it takes skills to truly maxmize the utility of the spy.
When the spy starts out from his home base, he must first start to figure a way out to sneak into the other half of the map, in order for his disguise to be successful (You wouldn't think the enemy is so dumb as to believe their own ally is running back from the other half at the very start of the game right? Same concept applies to the fact that it is somewhat suspicious to see a fellow ally running back from the enemy's half unscathed, at any point of the game). This is where cloaking comes into use. However, do take note when cloaked, you will still be blocking another player if you should cross his path, and the enemy is able to catch a glimpse of you if you are too close to them.
If you were to successfully infiltrate into the enemy's half, that's where the next challenge comes -- what disguise do you use? And where do you uncloak yourself? The latter is pretty a no brainer question; of course you should uncloak at relatively isolated corner such that no one sees you uncloak, or else your cover will be blown. How about the disguise then? What class do you impersonate? That'll depend one how are you are into the enemy's half. If you are relatively near the frontline, where soldiers and heavy (and perhaps medics too), it's definitely recommended you disguise as one of them. It would be odd to find an engineer or sniper running up so close to the frontline. Same applies if you are relatively at the back, where engineers and snipers usually hang out at.
It's a pity there's no friendly fire in TF2, hence people usually are able to check whether one is an ally or actually a spy in disguise just by shooting them. However, I have to admit if FF were on, the spy would be an overpowered class. Take note that the Pyro's flame will also be able reveal a cloaked spy, so don't get into a pyro's line of sight. I really enjoy playing as the spy (it's the class I played most so far) and it feels satisfying to do a successful backstab and then cloak myself, only to return later and kill another enemy. It's even more satisfying to have an enemy medic heal me, with me "repaying" by giving him a knife on the back.
Disguising myself as a Red soldier, preparing to jump diown and backstab the pyro
Cloaking
