UT2k4 is pretty much the best of the newest fast-paced shooters, and certainly the best in the UT series (excluding Unreal). While UT99 remains forever playable, there are several important differences between the two games, some which hardcore UT99 fans may not approve of -- but ultimately there is something everyone here can appreciate. Even Quake fans.
Firstly, the game modes. Apart from the typical UT99 modes, we have Double Domination and Onslaught. I was a little disappointed at the complete removal of 'classic' Domination, but the new mode does truly pump up the tension -- you now need to hold two control points not one, and only for ten seconds. Onslaught is a curious beast where you fight over control points in sequence complete with an array of vehicles. Naturally in this mode, if you're not in a vehicle, you're usually plastered in a pretty short order. I've found that this game mode suffers from that which Tribes 2 did -- you need an organised guild/clan to really make the best of it. Public servers of onslaught (which aren't that common) generally are wildly imbalanced. The handful of assault maps are pretty ambitious, even more so than the UT99 ones, and they're pretty good. But again, you won't find that many people playing it. UT2k4 does however, absolutely shine in the "raw" game modes, and this is one big benefit over UT99 in my opinion.
UT2k4 is fast. It's not Q3/CPM or even QW fast, but it's faster than UT99. Much of this comes down to the movement changes. Dodging, rather than a novel or useful jump feature, is now a critical part of a good player's movement especially combined with the new double jumping and wall-jumping. In this sense it's got a little element of Quake 2 about it all; you can get to places very rapidly that you otherwise couldn't, and very obviously outpace players who can't do it. Some people hate this, as UT99 movement was in many ways an order of magnitude slower, and hence it can be claimed a stronger emphasis on tactics perhaps. I'm not convinced personally. If you simply cannot move fast, you have to aim better. UT2k4 demands you practice movement and aiming, besides all the usual weapon tactics. For this reason alone, UT2k4 now makes a wonderful 1v1 game on the right map, if you're into that sort of competitive gaming.
The bots. Actually, the bots are also very good. They're the best all round AI FPS players I've played against, and have a wide variety of skill levels (just watch the jump between 'Skilled' and 'Adept'!). They'll time items, use shield-gun jumps to get to the tricky items, chase you damn persistently, and they really know how to use the guns. It's impressive. They're not without their faults, of course. As with all bots that don't have "hard-coded" maps, a few of them they really suck on. Morpheus 3 they generally end up with negative scores on after falling off countless times. Curse 2 and others they often get stuck under lifts. On Phobos 2 they occasionally manage to get to places where the waypointing just fails totally and they sit there for the rest of the game. On most maps, they'll give you a good game.
There are good maps. There's a lot with the game, and the maps are pretty much all gorgeously detailed. This was my biggest problem coming from Q3/CPM. So many polygons meant I had real trouble telling opponents from vegetation at a distance! You'll get used to it, and you can also force bright/teamskins to help you. Out of all the maps, there's a good handful you'll end up playing. As with almost every FPS out there, I wish there were more really playable maps, but there's always favourites, and at least the list is greater than 'q2dm1'.
The guns are nicely balanced, and it's always good to see since they're the meat to the game. There's nothing really new here. The old automag-esque starting gun is gone in favour of a little assault rifle. This seems pretty weak until you realise the secondary mode shoots grenades. With the right timing you can really surprise an opponent with those. The razorjack is gone. The ASMD is as mean as ever, as is the flak cannon. The sniper rifle has been toned down from UT99, which is probably a good thing, as I frequently found myself wading into melee range with it in the previous game, racking up the headshots. Most of the time it is replaced with the lightning gun now, which looks awesome (and sets heads on fire). The redeemer is still there. That said, for the most part you'll find yourself sticking with the ASMD and flak for close to medium range, and the lightning gun for long range. Minigun has its place, especially in CTF game modes, but the rocket launcher and biorifle are almost strictly defensive spam weapons now (only 3 rockets per load in the launcher). A few guns are seen only in specific game modes, like the AVRIL (anti-vehicle launcher), sticky grenade launcher, and the spider mines. There's also an ion-cannon target painter for giggles.
Mods are not the strongest place I can comment on. I love the base game so much I very rarely delve into the "total conversions", but I stack up mutators in massive swathes. There's a lot of useful ones, just note that not all the UT2k3 ones work. A smaller mod I have tried successfully is the TDM4/CTF4 mod, which gives you the TDM and CTF game modes with 4 teams. TDM4 is pretty exciting (and all the game skins seem to come with 4 colours as a bonus) but CTF4 is a little madcap for me. Following what the hell is going on in that is a full time job. One TC I did try was "Alien Swarm", which turns the game into a top-down mutliplayer action-fest. It's a good mod, but one you must have friends with to play, and you really need someone to hand who knows the maps and the game reasonably well or it will be a lesson in frustration handling.
The game's atmosphere is also bizarre. Really. UT99 had a fairly dark, almost gothic theme to it all, whereas UT2k4 swaps dark for crazy pretty often. The maps go from military bases to idyllic islands, underwater labs to low gravity magical forests. The player models are no exception, which can be anything from cybernetic freaks straight out of some sort of asian horror film to space egyptians. Yes, indeed. "Quirky" is the best way to sum it all up. You'll like it, or you'll endure it. There's no Nali Warcow this time, though (in the base game at least).
On a technical level, while my GeForce 1 DDR really struggled with the game, anything newer, especially GeForce 4 and upwards should handle it trivially. I run it often on a Dell laptop, which has some slapdash SigmaTel drivers which run in to problems by default -- pick audio safe mode in the options if you have trouble too. The game is practically bug free, and I can alt-tab trivially, which is a nice thing for competitive FPS games when you're trying to organise things in IRC at the same time. I've never had a crash when not using mods.
With Quake 4 not making any in-roads into the heavy online FPS scene, UT3 failing to take off (there's still more 2k4 servers/players), and Q3 really starting to die down, the only real remaining contender in the fast-paced shooter scene is this 4-year old monster, UT2k4. Four years old it may be, but it has aged very well, and still looks pretty damn good today. If you come from a Q3 background like me, give it a few days to adapt, and I'll bet that you'll like it. If you're from a UT99 background (also like me), you'll have some heavy changes to get used to -- and it's not optional if you want to be a good player. If you're fresh to the series, then this is almost certainly the one to get. I do hope you enjoy.