I must be lucky, the Sig 556 I got has been fine, so was the AK74 I got from Century. The AK had a slightly canted front sight post, but not so bad that it couldn't be zeroed. Other than that, my only complaint was that I bought a 556 back when they were going for $1,500; last I checked they were going for under a grand.
Accuracy wise the best I've gotten so far was right about 2 inches at 100 yards with Wolf, but I just recently got my first scope and have only shot the Sig 2 or 3 times with it. It cycles Wolf just fine.
The Sig does prefer P-Mags, it had some issues feeding from the old aluminum M16 mags I had laying around when I first got it but as soon as I got a couple P-Mags it was flawless.
It does have a tendency to ding up the brass shell casings a bit, something to do with the way it ejects. It also flings them a good 10 to 15 yards away in a kinda random fan shape pattern. I've been mostly shooting Wolf steel case so this isn't an issue for me, but if you reload 5.56 it might be an issue for you.
The Sig 556 SWAT Patrol model I got weighs in at 7.5 pounds, the SWAT model with the longer handguards weighs in at 8.3 lbs. This isn't an issue for me since I use my CMMG 22LR AR for the fun tactical shooting stuff when I get a chance to do so. I've really only used the Sig for slow aimed fire at 150 and 200 yard targets. The weight up front with its piston is actually a bonus for that since the weight keeps the muzzle down and dampens recoil compared to a M4. Everyone who's shot my Sig has commented on the lessened recoil.
The folding retractable stock is nice and locks solidly in place. It comes with two different butt pads, the one in my field strip pic is the longer one.
The front handguard rails aren't for putting optics on though as they move around like the standard issue M4 handguards do. I've only used them for putting one of those vertical grips with the spring out bipod on.
As far as cleaning, the bolt stays very clean and cool as pretty much all the gunk stays in the gas block, gas tube, and on the piston. These parts are very quick and easy to field strip and scrub off. The Sig doesn't have an M4 style star chamber that's hard to clean out either, the internals are very easy to clean.
Obviously your mileage may vary.