I've had several good RMA processes. I had an issue with my KGPE-D16 Opteron motherboard that I sent in for replacement to Asus. Responsive service to the inquiries I made and they stuck to their time line. Apple was really good for an RMA: they sent out a tech to check out my system before determining that the issue was a motherboard. Prompt service and they held to their time line, though I wasn't too pleased about a few week delay to get in the replacement board.
Now for the bad:
ViewSonic lost my original monitor in the RMA process after several weeks. I did windup getting the same model back but it took them another three weeks (and another phone call to remind them) to get it back to me.
Diamond wouldn't accept a Radeon 7970 that stopped functioning a little after a year. They threw the whole fit about registration to cover a warranty so it never happened.
Samsung is supposed to have a life time warranty on their memory. Unfortunately the group responsible for server memory doesn't even have an RMA department. Their phone system is a massive run around as well as computer memory is such a small department I'd often get routed to the wrong area. Regardless, I have two 16 GB paper weights.
Dual Opteron 6376, 128 GB DDR3, Asus KGPE-D16, Radeon 6970
Mac Pro Dual Xeon E5645, 48 GB DDR3, GTX 770
Core i7 3930K@4.2 Ghz, 32 GB DDR3, GA-X79-UP5-Wifi
Core i7 2600K@4.4 Ghz, 16 GB DDR3, Radeon 6870, GA-X68XP-UD4