In reply to Ali:
I used a Deuter Speedlite 15. you should just about get all your gear in it. It's light and packs flat: it fitted in the water bladder pocket of my main rucksack and also in the front pocket of Lowe Alpine pax 30.
it has proper shoulder straps, good zips, bottle pockets & bladder pocket.
I think the shoulder straps would be the main drawback of the other sacks....like everyone always says, try them on in a shop with weights in!