Even as a noob (started playing the day it went F2P), I have to agree in part with Snunch. You only really need 1 spy and MAYBE one Sniper. I've seen teams with 3-4 spies, and I just die a little on the inside. I really only play the Spy if the following is necessary: taking out buildings, hard to reach Snipers, and Heavy and possibly his Medic. It's also pretty fun disguising as a Heavy and luring in the Medic; you can get a handful of back stabs if the Medic is with a small crowd if you're fast enough.
With that said, Heavies are so hard to take down by most classes that they could use a good knife in the back to get rid of them. I mean, they can take 2-3 rockets to the face and live. Pyros like me don't have a chance, even if you nab in the back with the Backburner; they turn around and mow you over, but they might die in the process if anything.
All that said this game is pretty fun; I love how you can just pick up a game in a matter of seconds, leave whenever you're not having fun anymore, and pick up another game. Since having people in voice-chat seems uncommon, think TF2 is more enjoyable than any console FPS (online) I've played. That may change as more people get it for free, but hey, it's not too bad right now.