I don't have any experience with DD, but have seen a lot quality fluxuations in the AR arena over the last 20 years that I've been paying attention. When Colt couldn't put out anything that worked, Bushmaster was the standard for reliability. Now BM is considered spotty in quality and Colt one of the top names. I remember a bit of scuttlebutt about quality and reliability when the M&P first came out, but they've steadily become reputible. I've been using Rock Rivers since 2004 and have had great experience with the two I've run - one just past 10k rounds with no malfunctions of any kind that were gun related - bad mags and bad ammo on the other hand will jam up anything. The lack of being milspec hasn't hindered the RRAs to deliver good performance when properly maintained - just like you'd maintain your top shelf AR when running hard. If the M&P will accept aftermarket barrels, triggers, BGCs, etc then it's as good a platform as any to build on. Take a look at advertised prices of used ARs, and you'll not see too much depreciation. You'll likely find someone who will pay your asking price or close to it as long as you're not being ridiculous.
I'm an instructor, and of the M&Ps that have showed up on my ranges, they have functioned reliably, including the Sports. The only major issue I've seen was operator induced when he set his QD magnifier to flip right instead of left , blocking the ejection port and causing a huge malfunction. Although I've seen a couple of brands that were pretty unreliable, the M&Ps have run pretty good.
