My only experience with an ATI RMA was on an X1800XT with Sapphire. They charged a $15 fee just to look at the card. This is over and above shipping costs to them. It was a bit of a pain to get in touch with them and get the RMA process started as well. Worth it for a defective high end card, but on a midrange or lower it may be more cost effective to throw it away.
Now granted, this was early in 2007 so things may have changed since then.
The only other video hardware RMA experience I have is EVGA for a failed 7600GT fan, done mostly to see how they fare. It would have cost me less to just replace the fan myself. That was the most positive experience I've ever had with product support, and I have 0 qualms recommending them as a vendor. Too bad they don't make ATI hardware...