Owned two gen 2 556Rs. Wish I could have kept 'em both. Each ran flawlessly.
Tim (Military Arms Channel via youtube and full30) had lots of issues with his later gen 556xi's in 7.62x39. Although I've talked to a few who have had no issues at all with theirs, and swear by them. I have a 556xi in 5.56 and it is smooth. I think all of the 5.56 xi's were solid, though. I don't hear about any problems with them.
I would recommend asking to shoot before you buy (you supply the ammo) if possible.
FWIW-- I have purchased from this gentlemen before. He sold off a large collection of 556 / 551 clones and is now selling his remaining 556Rs. He was great to work with, and I would trust buying from him again. His prices seem reasonable vs gunbroker...
WTS: SIG 556R Rifles in 7.62x39 - AR15.COM