I've been using these mags for a couple weeks now and here's a quick rundown of the new 40 round Gen M3 PMag from Magpul...
-100% reliable in all of my riles thus far.
-Compatible with HK 416, SCAR 16, SA80, MK 16, Tavor, ect...
-According to Magpul, more durable than Gen2 MOE PMags
-Same marking system as all Gen M3 PMags
-40 rounds is obviously cooler than 30 rounds
-Compatible with the Maglink
-Firing from the prone
-They're tough to find currently
-Currently only out in black
Here's a video review with some shooting, a couple drop tests while loaded, and a discussion/demonstration of the pros and cons:
Magpul 40 Round PMag HD Video Review Link