It's not quite a belt system, but I recently acquired the Think Tank Speed Freak hip-mounted camera case
http://www.thinktankphoto.com/ttp_product_SpdFrk.php
I've only used it twice now (once while on my feet for 12+ hours at a motorsport event, and the second time with family for a few hours at the local park), so my thoughts are still preliminary (I'll post a comprehensive review after a few months use).
Compared to the LowePro, Tamrac or Temba products that I've used over the years, the material and build quality and design details of the Think Tank are superior, which is to say first rate. I suppose it should be since you pay a price premium, which I was willing to do if this was THE holy grail of bags.
Having used backpacks, shoulder bags, and neverready cases, I wanted an accessible bag that allowed for mobility and comfort. Thus far, the Speed Freak has impressed on all points.
Though Think Tank has smaller (Speed Demon) and larger (Speed Racer) hip-mounted bags, I wanted something that could carry the following equipment:
Non-pro EOS body (5D or smaller)
24-105L
70-200L 2.8IS
550EX flash
50/1.8 or 28/1.8 & 1.4x TC
The Speed Freak can hold the 5D with 24-105L mounted, the 70-200 with lens cap and reverse-mounted hood, and either the flash or one prime lens plus the teleconverter (as the flash is too tight a fit, I'll probably get a separate case that mounts on the belt). It has an additional zippered pocket in the front with myriad smaller compartments, and stretchable side pockets (mesh on the outer pocket, thinly cushioned inner pocket). It also comes with a cushioned compact memory card holder (holds 10 CF cards), a rain cover, and a very comfortable and secure shoulder belt.
Accessibility is excellent. I can wear the bag in front or quickly swing it to the back (for when I'm on the move). The side pockets are great for quickly stowing lens caps, batteries and CF cards. Changing out lenses while standing was a breeze. The first time I used the bag was at a race track in cold, wet, windy weather, so I got to test the rain cover, and it worked well. However, I did not use the shoulder belt, and after several hours, all that weight caused the top of the bag to sag outwards uncomfortably around my hips. Having now used the shoulder belt in conjunction with the hip belt, I find it balances well on my body, and I can still swivel the bag to the front and back without difficulty.
While I can zip close the bag top with the 24-105 mounted to the 5D (and the 70-200 in one of the side slots), it comes close, but is not possible to do while having the 70-200 mounted to the camera (I presume it's possible on the Speed Racer, but the bag gets quite big) . I can still use the center slot for the 70-200 mounted 5D when I'm at rest.
If you haven't experienced a belt system, I would encourage you to give it a try. I'm very happy thus far, but give me a few more months to determine whether this is the holy grail for me.