Hi Ray - I've had a Fatboy for a bit over two months now. I chimed in on the Proteus pack thread:
Proteus/Fatboy
Basically: I'd say the Fatboy's practically bulletproof as far as quality of materials and construction - the best quality pack I've owned, with a nod to the Gregory "Reality" backpack.
My main complaint, if you could call it that, is that it's smaller than it appears in the first photo (on the guy's hip, at that angle, looks like it will fit notebook-sized items, which it will not). The sizes listed in Maxpedition's ads are accurate, but when I was picturing in my mind what would fit in this bag, I was much more generous in my imagination.
What WILL fit in main compartment: I can fit a "mass paperback" - the Aubrey/Maturin series by Patrick O'Brian; in fact, I can fit two of this size book in it. Netflix return envelopes JUST fit. An 8.5x11-inch piece of paper, folded in half (top to bottom) will fit, in the length/width dimension.
What won't fit - full-size hardbacks, larger-format softcover books, my lunch, a change of clothing.
There's a smaller zippered pocket attached to the main compartment, also under the face of the main flap...has mesh divider, space for a PDA plus some room, or a walkman plus some room, but probably a bit too snug for a CD player (might fit - I don't have one).
The cell phone pocket snugly holds my Kyocera phone.
My phone's size - but it would probably swallow a folding phone.
I wish it were cut a bit more generously, maybe with the option to snug it down a bit, so I could keep the phone in the sheath/belt clip I usually carry - as it is, I have to carry the phone without that sheath. Also, has loops for two pens - REALLY fat pens won't fit.
The flashlight/knife sheath below the phone sheat will fit (probably) any 2x123, 2xAA lights, and maybe 3x123 lights (I don't have a 3x123 light). I do have a Pelican M6/McModule, and it fits fine, even with the brass tab.
Opposite the flashlight pocket is another zippered pocket - I've got a Leatherman Wave and a 4xAA battery carrier and an Advantix film case with 2x123 spares in it - a bit more could be squeezed into that pocket.
The key clip, which is very nice, is not something I use. It's behind where you can easily reach it when worn as shown in the first photo Ray posted, but you can very easily pivot it around and reach it - and if you wore it on the other shoulder, it would be easy to access.
There are two attachment points mentioned - one right under the keykeeper, the other on the outside of the zippered pocket opposite the flashlight pocket. I have a Maxpedition M-1 pouch (their medium one) on the attachment point under the keykeeper. It slightly brushes against the main flap when I open it, but not very noticeable. I use the other attachment point for a water bottle holder.
Also, there are a few odds-and-ends smallish zippered pockets you could fit some paper, business cards, small items in.
My main complaint is not being able to fit full-size books or notebooks in it - but it's hard to complain because the pockets are the dimensions they said they are. That said, it's of excellent quality, and I've stopped lugging around a lot of stuff I had in a larger pack that never got used. If they made a version with exactly the same quality but big enough to haul a notebook/magazines, full-size books and a lunch or gym clothes, I'd buy it in a heartbeat.
I got mine at countycomm - there are no retailers carrying it around me, or I'd have tried to get a look at it before I bought it. I think more outdoor stores should carry them, because they put most other packs of the same size to shame.
Dave