About a 20fps gain in heavy, varied texture settings on my system. Full load dalaran went from 40 fps to 60+
Specs: core i7 920 (oc'd 4ghz), 6gigs ddr3 ram, evga nvidia geforce 8800GT-KO (512mb)
It DOES help, but you need to not be GPU limited in the first place to see any sort of benefit.