I recently had a good experience with Gore, the maker of Gore-Tex, and just wanted to share it. I am not affiliated with Gore in any way.
I had a pair of Vasque Breeze GTX boots, which have Gore-Tex XCR liners. They were waterproof for 2.5 seasons of light to moderate hiking, and then suddenly not waterproof any more. The boots were otherwise in excellent shape, with many more miles left on them. I tested them in my kitchen sink, and verified a leak.
I had purchased them from Eastern Mountain Sports (EMS), who have a return policy similar to REI (take any return, anytime, for any reason). However, I had reservations on simply returning the boots to EMS, because I think they just lose the money. (As an aside, does anyone know if I had returned the boots would EMS get compensation from Vasque or Gore?)
Being familiar with the Gore advertisement of "Guaranteed to Keep You Dry", I thought I would see if it was true. I sent an email to Gore on their website, explained the product failure, and requested a refund. A day or two later I got an email from Bill at Gore, who requested that I mail him the boots for testing (at my expense).
I mail him the boots (along with my original EMS receipt), he confirms the leak, and asks me what kind of boots I want. I believe Gore will typically send you whatever Gore-Tex boots you want as replacements (any brand and model they can get their hands on). However, Bill later realized that I already asked for a simple refund, so that is the route we went. If I had requested new, more expensive boots, I don't know if I would have had to pay any difference in cost (I suspect not).
Bill tells me that my refund check will arrive in ~3 weeks, and it did. They refunded my original purchase price including shipping (but not the shipping cost to Gore).
So while Gore-Tex did not actually keep me dry indefinitely in this case, Gore did hold up the financial end of their guarantee. Knowing this will be especially handy if I purchase Gore-Tex products from stores with more strict return policies than REI or EMS.

