UT 2004's hectic gameplay and non stop action make it one of the best shooters you can buy for the PC. The graphics are great and very customizable so they can be downgraded significantly to play on older machines or upgraded significantly to look beautiful on your shiny new gaming rig.
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.