I'm using the TurboGL on my system (G400 MAX, Athlon 550, 384MB of RAM) with UT. Tried it and liked it.
Pros - Runs smoother. I've got the HUD on, the detail on the skins turned to medium. I think the rest is pretty high. Average FPS is about 45 at 1024x768 (anything less is uncivilized).
Cons - MEMORY HOG! I've 384MB of RAM. I used to leave HL up in the background. I can't anymore - it'll lock the machine, or make UT run like a flipbook.
Gamma's extremely low. I maxed the brightness, and it seemed to be as bright as the darkest D3D setting. I had to up the gamma in the display control panel. If anyone knows the console commands for upping brightness, I'd appreciate it.
I sometimes get black squares where smoketrails should be, and pieces of polygons are missing occasionally.
Just my $0.02