In reply to Fast Eddie: I've a Karrimor Global. Has an enormous capacity at 70 litres, and can also expand to 90 litres. The back system is fully adjustable and very comfortable, but to be honest I've never walked more than a mile or so with it.
The detachable daysack is a reasonable size, very versatile with shock-cords and pockets on the sides that can hold a one litre Sigg bottle (but will fall out when you try to leap across a cliff in Cornwall when you realise that you're not twenty anymore).
I think it may also have a lifetime guarantee.
The missus has a Karrimor Airport. I think it's 70L, but the back system is very basic; good detachable daysac, too. Main advantage is that it does have wheels and a handle, which may make you extremely smug as you effortlessly wheel it round an airport terminal, whilst your too-tight-to pay-for-a-trolley partner lugs a 90L sack on his back.
Hope that helps.