Logitech have great customer service. It's almost no questions asked and I never hesitate to order anything from them.
I've also had experience with Corsair, Asus, Razer, BenQ and EVGA.
Corsair are pretty decent and my replacement arrived quickly and without hassle.
Asus seem pretty stringent, but fair. They will check your product over to make sure you've not messed it up in any way.
Razer required a bit of persuading when the coating on my keyboard wore off within a few months of buying a keyboard, but they came through in the end.
BenQ were pretty crap and required prompting and reminders throughout the process.
EVGA were absolute crap and I will never order another of their products again. Submitted numerous support requests as well as an RMA request and the only one which was answered was to tell me that 110-120C+ was a perfectly normal temperature for an 8800GTX (not overclocked and in a well cooled case) and coiuldn't be what was causing my crash problems. Miraculously, the problem went away once I upgraded to a high end GPU cooler.