Also remember that liter ratings aren't always consistent from manufacturer to manufacturer, and the loading and construction style of the pack itself. My REI Lookout 40, for example, only seems to carry as much as other manufacturers' 30 liter packs.

Great pack, I love it, but despite being rated 40 liters . . . it's a large day pack, not an overnighter. Well, maybe one night. I actually like that about it, though. I got it my senior year of college because I wanted a book bag with an aluminum stay. Perfect for carrying 20 lbs of books to class, and has stood up to going on 6 years of use and abuse across three continents.