It appears that Magpul is taking baby steps towards the precedent set by the Surefire 60/100 round magazines. The Magpul PMAG 40 M3 will no doubt sell like 'hot-cakes'; especially with an MSRP of $19.95. Magpul has been toying with a 'quad-stack' magazine similar to the Surefire design for a while but time will tell if or when it comes to market.
I own a Surefire 60 and keep it in my HD gun. The only issues I have with it are it's heavy and pouch compatibility isn't great, but there's no such thing as a free lunch... I have 60 rounds on board with no reload required. The advantages I can see with the PMAG 40 is that it will be cheaper, lighter, and work in standard AR pouches. I can't wait to play with one!