Initial installation was hellish. 3dmark wouldn't complete and had a 10% FPS decrease in opengl applications among other problems (see my thread How to completely uninstall detonators in WInXP.) All 3D applications were very dark. I had texture flashing as well. I haven't been able to install a 40 set over 30.82 sans those problems.
40.71 beta on a virgin system produced more satisfactory results. Games (Q3, UT, UT2K3) are still a little dark, but bumping brightness a notch in games solves the problem without washing everything out. I had minimal FPS decrease in OpenGL (about 1%, well within typical fluctuation) 3Dmark2001SE improved from 10765 to 11661 with most of the improvement coming from the nature test. No texture flashing, crashes or incompatibilities as of yet. Here's a link comparing 30.82 with 40.71 in 3Dmark on my system.
MSI K7T266PRO2-RU with 3.3 AMI Fullraid Bios and PCR tweak 6B=F0.
Visiontek Geforce 4 ti4600, not OC'D
AMD XP 2100+ Not OC'D
2X 256MB Crucial DDR RAM @CAS 2
2X 60GB Western Digitals in stripe array.
Sound Blaster Audigy MP3+