Pure genious.
There are so many forms of gameplay in UT2k4. CTF, bombing run, assault, onslaugh, deathmatch, team deathmatch, and some more which I can't remember right now. All these forms of play really add to the UT2k4 experience.
The graphics are pretty nice, even if they are a bit old these days. I can run it at 1600*1200 resolution (higher if my monitor supported it), maxed out settings, and 8xSSAA with 16xAF. I can't do that with some games out of that time period. It just shows that this unreal engine is very efficient.
Sound is nice, but nothing like FEAR. I love it when the commentator makes all those commentating noises, where he sounds like a 7 foot tall man who's been smoking his whole life (in other words, a really deep voice). Those sound commentations are soooo funny.
There pretty much isn't much to the single player campain. You are part of a team in the UT universe, and you compete against other team. You have to pay your team members after each match. Other than that though, you are pretty much just playing games with increasing difficulty.
The only down side is that it's a 5.5GB game, without any mods! That's huge! But the game is very optimized, so I rather it take up that space then have worse visuals and frame rates.
This is a great game, and you MUST get this. Do it now. NOW! Or wait for UT2k7, but that may not run on your machine, depending how old it is.