Are you referring to the QD barrel uppers, or the modular lower?
I saw/played with one of the frist generation QD uppers a few years ago. My impressions were that it allows for a fast barrel cahnge, but the gas tube is very vulnerable- probably faster to just swap to a new upper. The only other complaint was that I have a tendency to grab right at teh magwell, and the QD barrel levers were rgith in that position- they wouldn't unlock or anything, but they have all sorts of ridges and nubs that kind of got in the way of my offhand.