I just recently purchased an SX-AR. A few thoughts:
It looks to be, on the inside and outside, an FNAR. There may be differences in the workmanship or parts used, but it breaks down exactly like an FNAR. It is also at least $400 cheaper looking at average prices on line and at gun shows and local shops. Got mine for $1000 from Gun Broker.
It is more accurate than I am. Shoots very tight groups. Recoil is pretty good. Shot 40 rounds at one sitting with just a little soreness. Very nice trigger.
It is more laborious to clean than any gun I have ever had, but still can be taken down, cleaned and put back together in about 30 minutes.
The included instructions are pretty worthless. I have pretty much learned how to take it apart by trial and error and by looking at forums and a pretty good Youtube video (just search "FNAR cleaning").
When first cleaning the gun, I accidently tightened a screw that regulates the gas system. Bad move. When first shooting it, it wouldn't eject any loads. Found out on the web and by calling Winchester that that screw shouldn't be messed with. They also said that I would just have to loosen it a quarter turn at a time until the gun would start ejecting again. Pretty frustrating, but I have loosened it and will post again if that works.
The forestock fits pretty tightly on barrel and I have, very frustratingly, rubbed off some of the camo on the barrel. It barely shows, but still...
Yes, I also have an SX-AR and love it. I have an FNAR as well and the guns are identical with a few small cosmetic exceptions. The Mossy Oak Brush is a nice touch I think. I am shooting 4 to 5 inch groupings at 575 yards with no problems.
Winchester SX-AR with Bushnell Elitte 4200 6-24x40mm Optics.