i have the seal line urban backpack in small. i love it. the back ventilation is good (though not perfect- you need to have straps adjusted right so that the ventilation pad sits right on the upper portion of your back as you're leaning to reach your handlebars),it has lots of adjustable straps and the closure system is really clever, functional, and simple.

the inside is just one empty hole and the front pocket doesn't hold much so you might want to spring for their organizer pouch if you have a bunch of small stuff to carry.

i use it every day on my commute to work and it holds a pair of sneakers, jeans, shirt, gmat textbook, two u locks, a small tool bag, bike pump, trixie tool, and some other stuff. perfect size for daily use but now i kinda wish i got the large for those days when i have to haul a bunch of crap (the large is over 2x as big as the small however so i dunno if it'd be practical for daily use).

i have no stability problems when using the chest strap (never really need the waist strap), even when spinning down a large hill at 120rpms+

over the shoulder visibility is good- you get used to looking around your side more than over your shoulder (the top of the bag sometimes obstructs that view a little but not by much)

its also on sale at ems.com for right now something like 88$. ive gotten a lot of compliments on how the bag looks.

when shopping for a bag, i compared this to the waterproof ortlieb and the freight backpack. i chose the sealline because of price, simplicity (not so many straps everywhere), back ventilation (though the ortlieb has it too), the unique closure system (i like that there's no velcro involved) and cause i thought it looked the best.


edit-- if you're not planning on using this backpack while on a bike, the back ventilation won't work properly since your back will be making contact with the non-ventilated lower portion of the bag. hopefully this review will help someone looking for a bike-specific bag