Having tried them all, Spider-Pro is the undisputed King. I stand by every positive comment made about them. I also have the twin holster belt.
I can RUN while wearing it on with a 24-70 f/2.8 on one side and the 70-200 f/2.8 on the other, both on 1-series bodies with my hands free to carry a third body with a 300 f/2.8 on a monopod.
Come to think of it, I can't recall ever reading a negative comment about Spider-Pro.
The one thing I don't like about my spider is the click-clack sound it makes if you run... I have a hand on the body at all times to avoid that.
Can it take a 400 and 600 on series 1 as well?
If you can carry it on a black rapid, you can carry it on a spider. It mounts the same way, and if anything, mounting those will only worry you about the torque on the mount.
Best part, the weight will be distributed across the hips, so you'll feel it less than when its mounted across the shoulders
I cant quite visualise this - the weight has to be supported from going downwards. My BR supports this downward force off the shoulders (I have the dual strap). My cameras hang at about waist level and with the large whites attached they hang horizontally (mounted on the lens ring). When I walk they are static because there are no forces to make them swing.
Surely if they use the pistol mount on the hips the walking will make them swing with the hips. Doesn't sound too comfortable with a 1d4+400 f/2.8 on one side and a 1ds3 +600 f/4 on the other. Surely supporting the weight there will be tiring?
Well, when you support off your shoulders, your backbone takes the hit. If you're carrying one, its a big harder as your weight will be towards one side, which will put quite a bit of stress on the backbone. You've got a dual strap so its not too obvious, but when you move the weight to your hips, your stress is taken up by the hips and the femur - easily the strongest bones in the body. I barely feel the weight of my camera + lens + flash, even with my colleagues 70-200 2.8 IS II mounted.
Its odd, I know, I thought the same about it before, and stuck to my Crumpler neck strap. All fine and dandy, but began to feel that I ought to get a BR strap. As luck would have it, I realised the guys I shoot for use Spiders. Don't ask me why I never noticed it before, but they did. I asked about it, and about the issues I had from watching the video's on youtube, with it bouncing all around and stuff, and they offered to let me try it.
No doubt you might say they are static as no forces make them swing, but personally the BR straps I tried swung way too much for my comfort. The Spiders don't swing as much, due to the way they hang - theres no allowance for swinging at all. Some bare click-clacking, but you can live with it. They hang pretty much the way a BR camera would hang - lens horizontal and facing backwards, but the good thing is due to the spider plate, it doesnt twist. Spider plate makes a hell of a difference over the clip used in the BR, imo, and makes it a hell of a lot harder to snatch.
That said, back to the topic, if I hadn't tried the spider, I would easily have gone with the BR. Its still a brilliant system, just that I find the spider fits my needs a lot more. Also: looks cooler =P