I've been an on-and-off hardcore UT player for about 2-3 years now. I really like the UT 2k3 demo, after playing it for about 4 hours (although didn't completely at first).
When I first got the demo, I though "my god, the graphics are incredible" .. but I had this sinking feeling that it was nowhere near to as good as the original. There's also the harsh realisation that you are nowhere near as skilled as in regular UT, and I think this is why a number of hardcore UT players don't like the new UT 2k3 ...
After playing it for a few more hours, I can say that I think this new game is awesome. The gameplay is very good, and the guns have indeed been re-tweaked. Oh yeah and adrenaline is your daddy (maybe hotkeys for the commands would have been better, but this way n00bs don't know how to use it, and they get 0wned even more .
I was a big-time user of the shock rifle, flak cannon, sniper and minigun in the original, and they play very differently now. One funny comment I hear about the new UT is that the shock rifle is useless, the ball goes too slow, etc. This is rubbish! It's still amazing, but even harder to use now; I have practised with it for a couple of hours and it's very very good. It does not need any tweaking at all, IMO!
One of the only flaws I can find is that I think the Rocket Launcher is a bit too powerful (all other guns are near perfect). The RL goes MUCH faster now, so it's way harder to dodge. I've seen much more RL whores in the game than anything else so far, but, again it's a demo, and they'll probably learn there are other guns to use!
Oh yeah and bombing run is your other daddy!