I played this game a bit last night for the first time ever.
I can see why people like it so much, it's very much a team based game: fast paced and tactical all at the same time. All the classes have their own unique way to fight, and if you play your class right you really help your team out.
The spy is cool, probably one of the most unique ways to play I have seen in an FPS. I kept trying desperately to get behind the enemy team and bust out the knife on some medics, engineers and snipers. I've gotten lucky with the pyro a few times as well, good fun indeed.
Only hard time I had was finding a good server, but I imagine I would have better luck if I knew the game modes better and played with the filters more.