Took it to the range Wednesday over my lunch break. Not a lot of time, and only fifty foot indoor range, but primarily a function test rather than an all balls out effort. Fired a hundred rounds...one box each of Winchester 115gr. ball and one of some crappy Portugese 124gr. stuff that I had laying around. All went "bang" without a problem. No failures to feed, failures to fire, squibs, short cycles, or anything else but consistent firing. Recoil is minimal, and accuracy was good. I had literally slapped a Tasco ProPoint (I know...shame on me, but all my Aimpoint Comps are on other uppers) on the flat-top just to have a reference for aiming. While not at point of aim (because of me and the red dot, not the rifle) my worst group was about 2 inches, and my best (with the Winchester ammo)was a ragged hole that you could distinguish individual hits, but they were all touching.
While the modified Uzi mag won't hold the bolt open, I think that the Colt mags would based on the design of the magwell adaptor (but then you have to balance out the cost of the mags versus the convenience of the hold open). I also concluded that because of the shape and location of the gas/brass deflector behind the shortened ejection port door, I can't mount an ARMS 38 on it. The RRA A2 tactical should mount fine because of its design, and I'll try a few others that I have laying around as well to see what will and won't mount up correctly.
All in all, I'm very happy with the upper.