I've sent a 22A and M&P9 back to S&W. Sent them an email describing the problem and pretty quickly got an email with instructions on shipping by FEDEX. They didn't ask me for any proof of original ownership. I've read that someone had problems getting them to repair a second-hand pistol, but, unless they looked up my registration with them, prior ownership wasn't asked (I bought both new). The 22A had a problem (firing pin) that would have been easier to fix myself, but I wanted to see what S&W's customer service was like. I was quite pleased, other than the dance with UPS on return.
I've heard very few complain about S&W's customer service and I've had one good experience with them.