Amazingly fun! They actually made this great game better!
When TF2 came out, I was a bit worried that it wouldn't be the shadow of the former classic. I was wrong.
This game is great! It's fun. It maintained the "cartoon" look without sacrificing the gameplay. The feel, the maps are mostly the same but better looking and the few minor improvements they brought in really *improved* the game.
The most controversial is probably the removal of the grenades. However, after a month of play ( I was in beta), I have to say that the game is much better without them (less spam).
If you liked TFC or anykind of fast-action team based FPS, you have to try TF2