That looks like a great backpack. I got a CamelBac years ago with the 2L water holder. It's small enough that I can actually fold it in half and stick it inside of my tank bag so I don't have to wear it all the time. The cool thing is that it has a spot for the water tank and the bite valve so you can drink while riding. The downside is that it's not that big. I just got a 15" Macbook Pro and I really doubt it'll fit in there as it's pretty sizable.
One of the things I hate about any kind of storage is that it can be bulky and sometimes you don't carry it with you. Kinda like my insulated jacket insert, I don't always carry it because of the size and I'd like to have a spot on the bike where I can fold it up and leave it there. I used to do that on my other bike but the 919 really doesn't have much storage.
I think you'll get a lot of use out of that one, it looks well made.