Honestly I've been running windows 7 ultimate x64 since before the release, and I've had no other major issues than my XP SP3 partition on the same PC - I mainly use the win 7 partition for everything now, except for mechwarrior 2, descent, and other old games that is. The built in 'Windows XP Mode' works well, but certian drivers dont work right, same thing like in virtual machine. So I use Xp for testing really. I havn't played MWLL that often on the XP partition, so I can't vouge for crashes or anyhting
I can, however, say that I've tried DX10 and DX9 on the WIN 7 x64 partition, and the difference doesn't really seem to be more/less glitchy, but just a gain in FPS from DX10 to DX9 - with my benchmark testing i've done, it seems that I get an average of 8fps more with DX9.
My system specs:
Intel DX58SO mobo
Intel Core i7 920(bloomfield) @ 3.21ghz (1.2v)
3x 1gb OCZ Platinum @ 666mhz. @ 7,7,7,14(1T) (1.65v)
BFG GTX 260 (65nm) w/896mb ram