Likewise, I thought the demo was "ok" but the full version was so much better, installed it shortly before my latest LAN trip and loved every minute of the entire day we spent playing it. The game feels so much more "real" than the original UT. I know it's all futuristic and such but it just has a polished feel that Q3 had, and now UT2003 has as well.
However, if I want vanilla deathmatch where the fastest finger off the button wins, I'll still look to Q3. If I want teamplay, I'm going with UT2003 (we had some incredible moments during the bombing run missions, true quality in a match where everything felt right. It was beautiful and once it was over everyone just sat back for a minute and grinned - we all knew we'd pulled together to win it).
Moments like them - they're what make games for me now.