I'm seriously considering one of these.
Any problems with them?

The earlier productions had issues with generation 3 pmags.  The issue was resolved and mine was purchased with the G3's in the package and I have not had any problems whatsoever. It is quite accurate for a 16" with the stock trigger and has eaten everything I have fed it.  Weighs less than the 716 and a bit more than the MP10 however you get the piston, adjustable gas setting if you want to run a can and all of the other goodies that come with it.  Great rifle for the money IMO.  If the generations 3's are in the package you should be good to go if it's a recent production. Someone else may be able to chime in regarding serial number prefixes however Ruger addressed this a while back.

Edit-  mine is 561-057xxx

Good to hear..  I am thinking about getting a SR-762 as well..  But good thing about Ruger is, they generally take care of any issues, and have good customer service IMO.  I had a SR-556 that had some FTF issues, shipped the gun in and they completely replaced....  

Zero problems with SR-762 (except for a heavy trigger pull).

To solve the trigger problem I installed a Giessele $45 patrol rifle single stage trigger that drops the trigger-pull to a crisp 6 pounds from the way too heavy 8-9 pound Ruger stock trigger.  Accuracy is 1 MOA with my best 150gr load and recoil is very light.  This is a very fine rifle.

The only issue I had with mine was that the carrier ended up eating the buffer face. Ruger replaced the carrier and buffer and had it back within 1 week.
Old buffer:


Old carrier


New carrier