If you're interested in hooking up with other players, we do have a Steam group on here.
Anyways, yeah, I love TF2. It is definitely a great game to play, the community is typically nice (save for the few trolls and such), it requires a lot of thinking and strategy to get right, etc. My main class is the Soldier, and I believe my max killing streak was 5 or 6 if memory serves me right, and that is excluding all of the assists.
I also enjoy playing as Engineer and Pyro, occasionally going for the Scout and Medic when needed from the team. As for the classes I cannot play as, I definitely can not play as Spy and Demoman, for my skills are not good enough for those classes, but I'm working on my Sniper skills. As for Heavy, I usually don't even touch him unless our team is doing horribly, we have an army of "battle medics", and no heavies.