OWC will definitely replace faulty RAM free of charge (example: we bought 2-4 sticks for each of 6 computers a while back, and three of the sticks were bad - very simple return process and they sent out replacements before the originals were returned).
I think the OP's point is that buying RAM through Apple (at a hefty premium) is a significant convenience for some people: if there's an issue - any issue - with the computer, take it back to Apple and they'll sort it out. Buying third-party RAM isn't quite as convenient.
Of course, in my experience Apple will happily look at your computer even with third-party RAM, but they obviously won't fix any issues caused by it.
OWC and Ramjet will both replace RAM. Ramjet has a page on their testing[1]. I have had great experiences with both companies.
Isn't that second thing the case with OWC RAM too? That it's covered by some guarantee and that if it goes bad, they'll replace it?