In general, the only issue I have with M1S kits is that EVERY BOLT CATCH needs a minute with the Dremel to open up the roll pin hole a bit; the guns don't hold open on the last shot otherwise, regardless of magazine used.
GOOD build: I built a Dissy kit from them back in 2004, and had no problems whatsoever; the kit was fantastic, and accurate, but front heavy, so I sold it. (Nothing on M1S, just the gun's style.) Maybe an A2 buttstock and a weight would've balanced it.
ALMOST perfect build: I later built another M1S gun, this time a kit with a chrome-lined superlight 16" with a standard A2 upper, and an ACEFX shorty stock. The gun is super light--LOVE IT! THIS one, however, had a problem--I think I got the wrong front sight base, as I couldn't get POI low enough to even sight in w/ the front cranked up/rear cranked all the way down. An extra-tall front sight post (and a nice upgrade to square, from round) fixed that for $15.
Planning another order as I type!