I purchased the small size messenger bag for my daily commute to work via train. I wanted a bag that was light, rugged, and would hold enough to be functional yet stay out of my way. This bag has been absolutely perfect.
I usually put my 13 inch Mac book in the rear compartment, a spiral notebook and my gigantic wallet in the main slot, my earbuds in the quick-access napoleon pocket, and cell phone and keys in the hidden zipper pocket in the front. However, when I need to transport bulkier items, it expands to fit shoes, boxes of food, clothes,and so on. There is a surprising amount of room for such a small bag. I've had water bottles leak inside the main pocket, and the lining really is waterproof - a quick wipe with a paper towel and everything's dry again.
The manufacturer does not skimp on materials; it's heavy-duty nylon backed by a waterproof lining, just like Chrome bags, and accordingly, it shrugs off the heaviest rain just as well. I would have no problem with standing outside all day in a downpour with my laptop in this bag.
As for ergonomics, the bag fits like a glove. Just snug up the strap and the bag adheres to your back without sliding around to your side.
Overall, I recommend this product because of the durability, security, style and color. It's probably the best bag to bring with you on your trip.
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