I have a Lowe Alpine airzone, think it's about 20litres, which I am really happy with.I also have an Osprey Talon 33 which is very comfortable as long as it isn't too full. My only quibble with it is that it is hard to keep the rain out of it. The lid is attached in such a way that it doesn't entirely cover the top of the rucksack, and it doesn't have an integral rain cover.