Trick construction, great durability, killer design
I've used this bag every day to transport a 13" laptop, camp seat, and several books every day in the rainforest. It is as waterproof as advertised, and the construction is waaaaay trick (everything well taped, solid drain along back and shoulder pad). The interior pockets are nicely sized for 2 32-ounce water bottles, and the tinier pockets are adequate for separating your wallet from snacks, digi cam, etc. There is a zip pocket underneath the flap that is slim and likely intended for a few papers. The zipper here is waterproof and hasn't gotten any easier to close in 6 months, maybe the only chink in the armor.
The branding and the pull down stays for the flap are all reflective (visible and subtly trick). I haven't tried out SealLine's attachments for the outside (either side of the bag or on top of the shoulder strap), but they offer two or so small bags for a cell phone. With all the anchor points, you could probably squeeze in 6 of the said bags... but you'd probably look like a tool unless you're a profession photographer or something.
My fiance has the smaller size and it's adequate for her 15" laptop and some other stuff, but it's definitely tight. My cpu is swimming in the large.... my guess is that you could fit a midsize 3 year old in it. Definitely will cover up your entire back in true bike messenger bag fashion (not to be confused with hipster/undegrad messenger bag style).
Other than the price and the lack of a truly padded laptop area (and the fact that BC.com only carries the green now), I can't complain. Some of my classmates have NF or mtn hardwear competition, and I don't think they're even close.