MUCH more on this at Mil-Spec Monkey.
Honestly, I don't think I'd trust my life to the thing in a firefight; I certainly have reliability concerns; but I bet threegunners everywhere are drooling right now.
Also, it's clearly significantly bulkier than the three mags it holds... in fact I'd guess its total bulk is around that of six mags (though not the weight).
Yes, there is at least some speed advantage there, and it's nice to be able to always index your hand back to the same place; but I'd bet that rangerplates or magpuls cut that speed advantage way down; and they add very little to the bulk of your kit.
The reloading motion from that indexed position is probably better in cramped quarters, but it's probably worse prone, so it kinda cancels out.
Oh and can you say battle rattle?
I bet it'd be spectacular vehicle mounted in a firing position on a bird, or light armor etc...