Kuba, sorry for the slow reply... I can personally attest to G.SKILL's RMA process. They actually went above & beyond in my case. I originally bought the 2GB Extreme PC4000 kit, and there was nothing wrong with it, EXCEPT that in my eVGA nF41 SLI mobo it's completely unusable. Wouldn't even pass memtest86+ at PC3200 speeds & relaxed timings. Dropped it in my other rigs, ran like a raped ape. The forums on eVGA's website recently were confirmed by an eVGA tech that certain memory types weren't compatable with the board. Of course, my memory topped the list of no-no's.
Long story slightly shorter, G.SKILL checked it out, & very quickly OK'ed an exchange for memory that was proven to work. I'm still pretty impressed by a company going that far to keep a customer happy. I got the new memory in about 3 days, and it's been running perfect.
As far as the warranty? All upgarde (performance) memory and all Extreme memory is lifetime warranty, the value stuff is 1 year.