Amazing.
Team Fortress 2 is possibly the best multiplayer FPS ever made. The first thing you notice when you play is the beautiful cartoon style graphics. Everything seems like it came straight out of a Pixar movie. Character classes (of which there are 9) are all very distinct and have their own funny and quirky personalities. This new humorous tone is carried over into every aspect of the game.
TF2's style is the only reason it is amazing though. It's gameplay is incredibly addictive. The game is perfectly balanced. Every class has a role whether it's gibbing annoying scouts with rockets, healing your teammates, or building sentries that make enemies quake in their boots. No class ever feels left out. The actual killing is also extremely satifying with guts and blood exploding with every death. It has an unbelievable level of polish too.
Valve is continuing to release more content for Team Fortress 2 constantly including new maps, new weapons, and more achievements. For $20, this isn't just a good deal. This is a must buy.