I have been playing UT2k4 for a few years now. Every time I step away from it for a couple months, I come back to find new and interesting content. The game has practically endless possibilities.
It has a bunch of different gameplay modes built in, and the user created content that's out there for it is simply just incredible.
Get it. Go online. Make sure you can see the servers running mods. Pick a server with a crowd and have a good time.