I've owned a Ruger SR556 now for about 6 months and have put probably 500-750 rounds through it (actually went to the range and shot it yesterday). It's been a very fine shooting firearm with no FTF, FTE or anything of that nature. This was my second AR (the first being a DI Bushmaster) and I really do love the Ruger more.
The internals of the gun stay cleaner; a good wipe down with Hoppes #9 cleans the bolt and carrier right up (no combustion gases in the receiver).
Ability to tune the gas system for different types of ammo (it has 3 settings and also an off position which essentially turns in into a straight pull rifle). I've found that on setting 2 it will shoot pretty much anything.
They designed it as a piston rifle so I have not seen any of the telltale signs of carrier tilt on my rifle.
It is a heavy rifle, I haven't weighed mine but I would guess it's at least around 9 pounds.
Price, with an MSRP of around $2000 for the non-E models it is an expensive gun. Although I got a great deal on mine as a factory refurbished model.