I recently purchased this bag and had hopes that it would be good for commuting. It is great for commuting. I wanted to ditch my backpack, I have done so. All that being said.... It wont easily fit a twelve pack of canned beer like my backpack would... but it will easily fit a 1L bottle of coca cola and a fifth of Jack Daniels. I have had it on the bike and have yet to have an issue. When expanded it can hold quite a lot of anything you throw into it. I have had it on my bike and packed with tools, water and all sorts of other things while riding some black diamond trails at hungry valley in Gorman. The Bag never flinched (even in a couple of falls, and the fact that I almost always have more weight in it than is recommended). It even comes with a Rain fly and a strap to wear it like a satchel! if you are looking for a little extra space...or are looking to ditch the backpack... I recommend this bag. there are only two downsides... #1 have to take it off to fill up (not that big of a deal, you are already off the bike) and #2, It makes using your rear grab handles harder, both to maneuver your bike and for a passenger to hang on if you have one. In my opinion, These are nowhere near deal breakers.