In reply to monkeys: I've had it pretty full on big winter days and it's never bothered me; I can't say I've ever noticed restricted movement, that's not to say that the problem doesn't exist- like most kit it's a personal thing: perhaps have a wee gander of online reviews, see if anyone else notices a problem?
I find it a really god piece of kit, anyway. The lid and the plastic back can be detached and the roll top main bag can be used on it's own, this cuts it down to a really light and fast bag for say summit days from huts or whatever.
So many bags on the market, there's the age old classic Pod (Black ice?) that's always been up there for winter climbing.
C