I've only had experiences with eVga, BFG and PNY. PNY did not honor a supposedly "lifetime" warranty because it uses a different definition of "lifetime", i.e. "lifetime" for PNY is limited to the product manufacturing cycle. Once a particular card is no longer manufactured, "lifetime" support ends. On the other hand, BFG replaced an end-of-life-cycle card for me with a newer version card which really performed better than the orginal. As for eVga, I've yet to have one of their cards crash, but then again, they are relatively new 8800GTSs, but indications are their warranty is as good as BFGs.
edit: I think PNY has gone to a new warranty policy. No more "lifetime" warranties, instead, one year and then an additional 2 years upon registration, 3 years total.