The 650 just stopped working two months after purchase on my main home pc, this february. It would light the LED on the mobo, but the fans stopped working and it wouldn't boot. I have been a frequent visitor on the Corsair support forums since then, and you will see many many 650s with similar issues.
The 750 was in my office and lasted for a year or so, this one burnt (that's what it smelt like), and took out the mobo in the process. I don't know the exact reasons since it Spain we usually do not RMA directly to the manufacturer (at least that's how I've always done it), the shop usually does it, and they get the replacement which they then give to you. The person that gave me my replacement didn't have the slightest idea about what was wrong. It did take quite a bit of time though.
Anyhow, I may have been unlucky but that is enough for me to stop buying them. On another comment, I had no idea these are made by Seasonic. Why would Corsair do something like that? Not like these are dimms or lcd displays....