UT 2004 is one of the best First Person shooters for the PC.
UT's replay value is virtually infinite. It has an online component that is just like the single player and it always has lots of people to play with and it is the most moddable game in history so there is a huge slew of user made mods out there for you to try. I suggest the ballistics mod. check moddb.com for that.
The only thing really wrong with UT 2K4 is that if you cannot go online, then it can get stale fast. The mods help this but not enough. The bots just don't act the same as human players and the online arena presents a much more entertaining challenge.
I suggest that anyone with a pulse give this game a try. It is a blast to play with friends.