I've been 'studying' AR's for a while now. I almost bought a Colt, a Bushmaster, a Smith and Wesson, and a sweet rifle from a small local manufacturer Arrow Arms. I saw this one in the Armslist and since it was a Ruger (and I love them), I did some reading. This is not a direct gas impingement operated action, like most AR's, but a piston action. There are a lot of piston conversions out there for gas action rifles, but this was designed to be a piston action gun. I like the look of the gun as well, and its stats are real good. You can read about it here or here. I also got a sweet deal on this one, about $400 to $500 dollars off what it is valued (with some upgrades and if it were new of course). It's got little use and looks new.
I've looked at literally hundreds of AR's, held dozens of them. I've shot exactly zero, so I'm not sure that I'm the one to give you any advice.. but I hope this is a sweet rifle.