In reply to alasdair19:
I second the 50ltr pack option for multi-day (e.g., The Ben for 4 days)...I guess I like my down jkt, big down sleeping bag, good food and thermal tights too much, but then again I probably wouldn't actually climb with it either. I just bought a Grivel Manu for that, stick it in the 50ltr pack, set up camp and then use the 12ltr for the climbing bit.
My Arcteryx 35ltr (with the rope carrying straps) is brilliant for light and fast stuff, though I can't get my sleeping bag into it!