ASUS has a pretty good RMA service, very fast response. very friendly people
MSI RMA is a little slow but their quite generous when they give you your part back with tons of extra goodies. (extra connectors, bridges, etc.)
Gigabyte is pretty cut and dry, nothing special about them.
As far as i've heard ASRock are the guys to watch for, give them a few years and they'll be some serious players among the big companies.