I have the jumbo, and I hate it, if only because it isn't ambidextrous and so I can't switch which side of my body it rides on. I finally turned it into a grab-n-go bag that I keep under my bed in the event of a middle-of-the-night earthquake.
The one thing that I really like about the jumbo is that you can carry a liter of water on it really well.
As noted above, an 8.5x11 pad doesn't really fit in the jumbo. A yellow legal pad fits into mine width-wise, but it's tight and the bag is currently empty. I wouldn't want to try it while the bag was loaded. Also, the bag is too short for a 11" tall pad.
I ordered a collosus, but it hasn't arrived. If you wait a few more days (I realize you started this thread back on the 2nd) I can give you a better review of it.
I already have a roly-polly, so I'm going to attach that to the bag for carrying water bottles. This is going to be my road-tripping bag. The fact that it gives quick-access to a concealed weapon is a plus (don't want the pistol on my belt when I'm in for a 8 hour drive). Plus the main compartment is padded, so it ought to make a decent SLR camera bag for those day trips to the zoo.
As for writing pads, I carry
moleskine notebooks, usually the small ones, and these fit into any man-bag that I might carry so long as it's bigger than a fannypack.
If you must carry a full-sized writing pad, and you want to carry it in a bag with a slimmer profile than the collosus, look at the Malaga gearslinger. The rear pocket has the same dimensions as the collosus, but from the specs I suspect that it's considerably less bulky than the collosus.
