Just a personal story, but I'm 16, and don't think I look much older. About a month ago I went into a Best Buy, and was able to purchase a copy of CoD4, without any issues. Sure it's not GTA 4, but it's still rated M, so they should still check my ID. But the guy even asked me if I was sure I didn't want to buy Ninja Gaiden 2 instead. Maybe the guy who got it down from the gamecage was just cool, maybe it's just that one store, maybe I do look a little older than 16. But it seems to me that at most game stores, restricting the purchase of M rated games is at most treated as a formality (I have to or I'll lose my job), and at least, treated as a joke. Or in this case, not treated at all.
But yeah, I don't think the purchase of M rated games should be restricted. I mean, the parental permission for younger kids is a good thing, but if you're old enough to get to Best Buy (or Gamestop or wherever) on your own, you're probably old enough to handle an M rated game.