First impressions:
I have a Pentium 4 3.2GHz with HT, 1GB RAM, and a 256MB Radeon X800 graphics card, and the only way I can get 50 FPS is to run the game at 640x480 on a CRT I've hooked up to my laptop, with all the settings at minimum. But, it does play! So while I'm planning to upgrade so I can max out my FPS at a more standard resolution, it'll do for right now.
The game is fast, faster than UT2004 in terms of movement for sure. It feels Quake-like at first, but the more you play it, the more you notice its distinct flavors. Rockets and Goo are back in a big way. The goo stays around a lot longer and has a larger explosive radius, and a single rocket will one-shot a fresh spawn. The item timings are nearly the same (55 for big armor, 27 for small) but there are now different small armors to collect: thigh pads that give 30 armor, a breastplate that gives 50, and a helmet that protects against one headshot (also gives some amount of armor, I forget how much). The big armor is now the shield belt from UT99. Most of the other guns work the same way. The minigun starts shooting immediately now instead of having a delay, but it does still go through a "wind up" phase where it shoots slower than full speed. The sniper rifle is back and feels like a decent compromise between the LG and the overpowered UT99 rifle. The infinitely abusable shock rifle from UT2k4 has been pared back to balanced. Shock combos do more damage but travel faster and the ball is harder to hit.
The graphics really are exceptional if you can run them, and the sound design is excellent this time around. They keep some of the memorable sounds from the previous games (like flak primary or shock secondary) but give some of the noises some much needed "oomph". The rocket explosion noise has a good bit of bass in it now, which I feel was much needed.
Most of the deathmatch maps that shipped with the game are not very good, but that's no different from UT2004. Standouts in my opinion include Rising Sun, Diesel, and Sanctuary. I haven't played through all of them yet though, so my mind isn't made up.
The one major irritation I have in UT3 is the lack of a "throw weapon" button. It really limits the amount of teamwork that can be accomplished in TDM, and it makes playing with WeaponStay a pain.
Probably the most major change to deathmatch play this time around is that when the player with the double damage is killed, he drops the remaining time as a powerup along with his weapon, Quake 3 style. This completely changes the dynamic of the powerup, as you can no longer suicide-charge into a guarded amp room and take it from a prepared team... they'll just take it back. On the other hand, if you don't prepare yourself well for the amp take, you might as well not guard it at all, and play defensively. I'm not sure if the change is good or bad yet, but time will tell.
Overall, an excellent game that was well worth the money, to me anyway. I haven't tried CTF mode, and I played one game of Warfare/Onslaught. The only comment I have about that is that the new vehicles are better than the old ones-- but they're still vehicles, and I don't like vehicles (just my opinion, different games for different people).