UT2004 is a "best of breed" when it comes to first-person shooters.
For starters, lets get the graphics and sound out of the way. Although the Unreal Engine 2 is not native DirectX 9 technology it is very flexible and highly customizable; and it shows because UT2004 showcases a large variety of game types, stretching from the basic Deathmatch to the new online favorite: Onslaught. There are over 100 maps to choose from and each and every one of them is carefully designed to offer the best and most balanced gameplay experience, despite not looking as 'unreal' as UT99's map selection. They do look great though, no doubt about that. And that's mainly because the engine is capable of rendering huge amounts of details such as foliage, particles or light glows without taxing your system's resources too much. The ambient sounds are also good and play a vital role in gameplay because you can get the upper hand over your opponent by listening for footsteps or power-up pickup sounds. And finally, as with all Unreal games, the music is in perfect sync with the action and will elevate your experience, it's that good! But if by any chance you don't like it, just listen to your own music via the built-in music player; the option in there.
But enough about graphics and sound. The game mechanics are probably the best thing about UT2004, they are simply among the tightest, most perfectly balanced I've seen in the genre. Old school Unreal players tend to say UT2004 was a step in the wrong direction concerning game mechanics because weapons are not that brutal anymore (not nearly as spammy either) or the movement is too fast or it's harder to hit targets. Yes, these facts are true but it makes the game experience much more engaging than UT. I admit it can be hard at first, specially if you play online but after you master dodging and double jumping (and use them constantly) things become a lot easier. You will see that by using these skills you will be able to cover huge amounts of ground in no time and be generally more efficient. Also, the weapons are not as spammy as the ones from UT99, the developers focused more on balance and they have achieved it. You basically have the perfect weapon for every situation, it's just a matter of you making the right choice at the right time.
Having said all that, I think you now have a relatively good idea of how the game plays. UT2004 can be either mind-bending fast, totally chaotic or even tactical, it just depends on what you are playing: assault, bombing run, capture the flag, deathmatch, double domination, invasion, mutant, onslaught etc. I'm not going to get into details but I will say that each and every one of these game types offer something different and interesting. The single player campaign is more complex now, it is not just a simple ladder mode. You have a budget with which you have to get along: first you draft a team, play through the qualifying rounds and only then enter the Tournament proper. Here you can challenge other team leaders to 1on1 matches for money or challenge a team for one of their players. It is fun and becomes very challenging towards the end so be sure to check it out before going online, it makes for good training. But yes, online is where the fun is and there are tons of servers out there so don't worry about not finding what you want. There is also an Instant Action mode if you just want to jump into some offline action.
The first time you will run Unreal Tournament 2004 you will certainly notice the huge amount of content this game offers. But you can find a lot more by checking out the various community web sites: total conversion mods, tons of new maps, mutators and new weapons are all available to you, for free! Needless to say UT2004 is one of those games you will not uninstall very soon, it deserves a perfect 10 when it comes to value.
There are tons of other stuff to cover, the game is just huge! But I'll end it here by saying this: do not hesitate to purchase UT2004, it is worth a lot more than you're paying for it, trust me on this one!