I have the MOE version, and it's been absolutely flawless, despite my tinkering with it. I used in in a tactical carbine match last weekend, and it worked really well, despite the overly dusty conditions and 100-degree heat. However, knowing what I know now, I would choose to build my own rifle from scratch if I had it to do again.
You really want a mid-length gas system, and a M16 full-auto bolt carrier group, a heavier buffer, and a better action spring. You can't get any of that in a factory S&W.
I'm assembling a cokmpletely new upper with a mid-length 16-inch stainless barrel, upper reciever and M16 full-auto BCG, and am trying to decide if I should float the barrel (it costs $100 more than not floating the barrel). At that point, the only original S&W parts on the rifle will be the lower receiver, buffer tube, fire control group. Everything else will have been replaced.
Parts list:
Blackhole 1/8 twist stainless Barrel - $165
Other upper parts (no floated) - $140
Upper Receiver/BCG $200
CTR Stock - $80
That's $585. If I were to get another complete lower, you can add $250 to that ($835), and event then, it still would have cost less than the rifle I originally bought at the local gun store ($1100).
BTW, stay away from the S&W ORC - they weld the gasblock onto them.