I don't have a "lot" of rounds down the pipe but I would say about 5-600 by now. Gun runs like a champ, I did find that it doesn't like junky mags, such as Thermolds. Every once in a while it will choke and fail to feed wit the Thermolds, never once with my PMAGs or the 10-rounder that came with the gun.
Only thing I can think of that's an issue is the notched hammer with the unshrouded carrier. It beats the firing pin to hell so much so that I'm debating buying a few pins and storing them in the grip. Everything else is great, cleaning the piston and is really simple and there's almost nothing to clean in the upper or lower receiver. Oh, and by the way, it's pretty hefty. I like the weight but some people would rather keep it light, that's up to you.
As for the upper to lower fit I'm fairly sure the Ruger upper works with a standard lower.
Run your friends SR556 a little more and decide if it fits your needs. I love mine. I just wish we had handguard options. I think this thing would be amazing with a Fortis Rev on it or something. Maybe one day.