I use OpenGL with Tribes, works fine, but that game is fairly limited graphics wise. I also tried the OpenGL render in UT, looked great, but was a bit slower than D3D, so I didn't stick with it. Don't know what sort of problems you are having, I think Q3 looks so-so in 16bit, but I don't know about other games (aside from those I mentioned). Some feedback back from Tribes2 (OpenGL) beta testers is that the Radeon rocked in that upcoming game.
-AJ