I have a 35l rucksack for walking (one of those stupidly overpriced things with the funny back that sits on your shoulders and hip) and I have had a 35l Kriega. For some reason I could get loads more stuff in the kriega than the berghaus - I think the point is that not all 35l sacks are the same!

The US20 (which I also have - actually I have far too much stuff!) is a great piece of kit alone. I used it in Portugal to keep all my clothes/personal stuff in for two weeks. I kept clean clothes in a roll-up-vacum-bag, and my dirty stuff in another to compress the air out of the clothes and keep clean/dirty stuff separate. The carry straps on the US20 are very good and you could carry it easily through an airport.

One of the other plus point for the USx stuff is that it fixes to the seat, so you are always going to be able to strap it on somehow, just take some good quality straps with you (I'm not keen on bungee cords, prefer good ol' straps).

Whatever you go for, you need it to be comfortable when full, so look for a good mounting system. My <stupidlt expensive> walking bag is superb as you don't know it's there. I have had some that were crap and left your back sore, aching or you could feel the contents of your rucksack digging in your back!

A trip to somewhere like Keswick / Ambleside is a good day out as you'll get a real feel for what you want as you walk around all the shops. You will find everything from cheap crap to overpriced stuff, and just what you need somewhere in the middle!

Consider some of the Army Surplus stuff too, some of it is crap, some of it is very very good.

The best thing is to get out there and look around at what suits you.

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Never underestimate the power of stupid people in large groups.