I have some. Probably 25L when strapped down to level. You can indeed leave them open/use the shoulder strap to extend the strap over the top. The roller plus ones are a nice improvement if you can find the old colourway, but not really otherwise worth the money.
have you got some? when they say 25l do they mean compressed? assume you can stuff a bit more than that and have the tops open if needs be?
thanks