I use an Arc'teryx M20, which is pretty sick. I find it too small, so I'm looking to move up to a 30L. If you want something for backcountry (ie that's gonna fit your shovel, probe, skins, bit of food and maybe an extra layer), 20L is gonna be too small probably. If you're just kinda hiking in bounds a bit, or like to cary lunch on the mountain, this is a pretty slick pack.