I bought this pack because I have been riding my bike for commuting for well over 2 years now and thought it was finally time to buy a bag that would be comfortable to bike with.  I read reviews for this and the 14, and thought the 18 would be big enough.  When it arrived, I loved, loved, loved it!  The design is wonderful and the bag is lightweight.  When I tried it on for size, it fits perfectly!  I can imagine riding with it comfortably.

When it arrived, it looked smaller than what I expected.  I realized that I needed to loosen the straps to get the same size as in the picture (I meant to take pictures before I returned it, but I forgot).  I decided to return it after placing my laptop in the hydration compartment (I never intended to use this bag as a hydration bag, btw) and finding that it was a snug fit.  I don't USUALLY take along the laptop (Dell XPS 13, I'm not sure how big it is, but it is slim like a MacBook), but I wanted a bag that would be at least a little flexible for packing.  The snug fit would not normally bother me, but I also carry around an iPad for daily use and trying to pack both with my regular gear would make it too snug to be able to get things out of the bag easily (although it is important to note that this may be good for biking, as there is no shifting around while on the move).  For commuting to work, I also pack lunch and a change of clothes, along with tools and usually a notebook or two.  After three hours of contemplating, I finally decided to return this bag and get the Osprey Momentum, which is designed for commuting, rather than hiking/mountain biking and is not a hydration pack.

I do love this bag, and I plan to purchase it as a second backpack some time in the future.  I could not justify the expense for two backpacks at the moment (since the Momentum is over $100), so I had to return it to get the right one.  If I am not planning to pack too much for a day (which I do a lot...:D), this bag would be perfect.  I judge that it could fit the laptop or the ipad along with my change of clothes, lunch and all the various tools I need for the day.  I'm not sure what I would use the open pockets for, since they are neither secure nor water resistant, but I'm sure I could find a use!