I have both the Fatboy and Jumbo Fatboy. For small to tiny shoulder bags they are certainly well made. However, the fatboy is too tiny to be much use if you carry a bottle of water so I would avoid it.
The jumbo has a 1L bottle pocket which is its best feature. The main compartment and other pockets are decently arranged but again sort of small. This one would work fine for daily carry of a small amount of junk.
Their shoulder strap arrangement coupled with a belt attachment is very good, the belt tab keeps the thing from swinging and can transfer some of the weight to your belt.
They are quite overbuilt and really not very useful shoulder bags for say a day walk, where you might want a camera, water, a jacket or poncho, and a few other items. All the accessory pouches etc that they sell once attached makes this an odd monster of fabric and straps with even more limited use in my view.
Try to see them in person as I was appaled at how small the fatboy really was...almost useless really.
The best online review of these two bags can be found over at British Blades by MattW.
http://www.britishblades.com/forums/showthread.php?t=13857&highlight=fatboyOverall I think the jumbo may be a decent bag for your daily carry needs, but I find it too small to be my first choice. I would rather carry a Filson field bag or a Billingham camera bag as they are way more versatile.
As to a BOB bag, I use inexpensive duffels or plastic storage totes, not BOB's really but go-kits for various purposes.
Now that I have these two Maxpedition bags (along with a couple of belt pouches) I do not think I will ever buy anything else from them. They seem to have a neat factor but for the most part a very restricted use factor. In other words, I find their form does not fit their function. All design....no practical use. Well made, but useless. Sorry to be so harsh.