I'm pretty pleased with a Karrimor Kimm 35 I purchased earlier in the year. It compresses well for smaller loads, it sits well on your back and doesn't interfere with your arm action. It was designed for Mountain Marathons, and cuts out all unnecessary weight. I've run with it almost full and with only a 1litre water bottle and it was OK for both.
You can check it out at: http://www.kimm.com/new3/index.html
It doesn't have padding for the back. This is OK, providing you pack it well. But it isn't like a familiar backpack that holds its shape when empty. There are no compartments and no zippy-up side pockets. The one I've got isn't made for an internal platty-bladder, but I'm not too keen on these things anyway. I believe the latest version does take one.
It isn't perfect, but then what is? Personally I'd add a bit of back padding - to give it a more rigid form. I'd also add a pocket or a clip which would hold a map when you don't need it.
It's worth checking out. You get a good discount at: http://www.wiggle.co.uk/ - where I bought mine, but I think what they have in stock is last year's version, ie minus the enhancements you read about on the kimm website.
