UT2K4 is a great, fast paced FPS. With both a single-player option and the ability to play online with many different gameplay modes, there is plenty to keep you occupied. While the single-player mode can be a little repetitious, it still offers a decent challenge and is a good way to get warmed up and learn the basics if you never have played before.
Multiplayer is where it's at though. There are many modes of play including capture the flag and deathmatch, and a few original modes too. Unlike shooters like Halo and Call of Duty the action is very fast paced and you have to keep moving to stay alive. Expect to be playing a long time before you become any good though.
The game looks very good, and there are plenty of customizable options. Also, there are many mods available for this game, giving it a lot of replay value.