This game is currently the best game out there for team based action.
The game forces you to use teamwork no matter if you're playing in a tournament or just in a pub, there is no way around it and that is the main thing that sets Team Fortress 2 apart from other class-based-team-focused-multiplayer games.
Its art style is/will age very well (now if they would just design levels with a few more colors...) the gameplay is constantly advancing due to Valve's consistent class and map updates, and the community, at least on the PC side is very strong and growing.
Overall if you're looking for a game that is fun, intuitive, unique, challenging and addicting, look no further than Team Fortress 2.
