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I got the STM small Loft, and I was impressed how much protection it did give. It's got pretty beefy firm padding (rubberyleather) along the bottom and sides. My macbook is very snug :) and I feel pretty safe when I put the bag down, like I bring it down without caring too much (as opposed to thin bags where I'd carefully, gently place it onto the ground or whatever). The top just folds over though so it would be possible for a bit of rain or snow to sneak in at the top. I wouldn't be concerned about that unless it was seriously coming down hard.
 
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