I have the original (2009? with gray insides) version. It works, and you can likely get it cheap. I wear the shoulder strap (has nice pad) and waist strap at the same time, and it's snug and the weight is distributed well in spite of large size and typically heavy load I carry. To grab a shot, I loosen the waist strap and swivel the bag around front with the weight on the shoulder strap. I can work from the open bag while shooting, changing lenses, and other fiddling. The open lid stays more or less flat so you can get stuff from the organizer pockets. I can have the camera ready in maybe 5 seconds. The rain cover is good for security in crowds and you can cram jackets and stuff in there if you're not planning to take a shot for a while. I use it hiking, cycling, scrambling, traveling. Unless you have a very very long torso, you probably will not be able to wear any backpack above this bag. I wanted a big bag with quick access and good ergonomics for all conditions, and this is one of the few options. I don't like sling packs (single strap backpacks) for long hauls, and having my camera out of reach in a regular backpack or swinging around on a neck strap is out of the question.
Devilish details: The waterproof main zipper slows me down. Worse: the shoulder strap buckles are sewn into the same seam as the zipper (I was tempted to cut them off but don't! see above). I often the leave it half-zipped. The lining is not very securely stitched to the bag structure, and it is kind of floppy inside until you get it loaded up. It would be nice if the inside velcro lining extended up a bit so you could get creative. I'd like a brighter rain cover and some reflective stuff for safety. All of the organizer pockets are kind of 2-dimensional and hold little. Webbing of the waist strap is showing wear fast, but it isn't slipping yet. I do not use the strapettes. They would really slow down access.
I see that in the 2010 model the main zipper is maybe not affected by buckles and stuff. A mesh pocket has been added. Generally I don't like these because they snag on branches and get trashed. Haven't seen it in real life. Can't be bad.