In my experience with a local brewer, the growler, at $9, is cheaper than the same quantity of beer in glasses at the bar, but not cheaper, ounce for ounce, than a 12 pack of, say, Boston Lager in the grocery store. Growlers are appealing to look at, on a picnic, staying cool in the river, but they're not the best deal, cost wise. I don't think people will buy less bottled beer in favor of growlers.
trader, agree but the volume is irrelevant in the grand scheme of things. With such a limited shelf life and large amount of effort needed to fill a growler this is a very limited, highly specialized market. Growler sales should have no bearing on packaging craft brewers volume.