I recently got a Thinktank Speedfreak shoulder/beltpack. It is more comfortable to wear than a shoulder bag, however I am spoilt with respect to comfort and easy access to equipment by the Thinktank modular components, which is my preference for longer excursions/hiking with a regular backpack. The disadvantage of the modular approach is that is a lot more work to get the components on and off (I prefer to use shoulder straps in addition to the belt attachment); thus my purchase of the Speedfreak for shorter excursions. It is not as easy to rotate front - back as I hoped, but it depends on of how much the belt is tightened. The internals should have come with more Velcro for attaching dividers, it is geared towards dividing the internals into 3 main compartments, so it is due for some modifications to add an extra divider between smaller AIS primes on on side. While all the ThinkTank components come with rain covers, I have never used them. The outer fabric is lined with an internal membrane that is pretty water proof, so there is just a little bit seeping in thought the seams.
I chose the the Speedfreak as a compromise in size; I am just able to stuff it into one of my Ortlieb rear bike panniers, so that I can have a good deal of equipment easily accessible on shorter trips with my bicycle. Which brings up the question how much equipment are you usually carrying in your bag -lenses/ one or two bodies? For most easy access the Speedfreak will fit one body, 300mm +TC on one side, One AIS prime + an adapter under the middle compartment, 12-24mm on the body, and 3 smaller/middle sized lenses on the right side. It is possible to stuff it with a few more lenses and my AW1 for travel/transport, but then access becomes difficult for the parts at the bottom. The front compartment has room for some filters/spare batteries and a blower. My brother has the bigger Speedracer, where he is able to fit a second K5II body attached to his 300mm lens, but for lighter excursions and travel he seems to prefer the Thinktank Changeup belt pack.