This review is for the Blackrapid RS4, Blackrapid Double strap, and the Cotton Carrier. I own all three. If all you need is one strap the RS4 is impossible to beat. It is comfortable and very convenient to use.
The double strap is the most comfortable way to carry two cameras (I carry a Nikon D700, 24-70, SB 900 on one side and a Nikon D300, 70-200 VR on the other). I can WALK around all day with absolutely no discomfort from the weight of the cameras. They won't slip off and are both very easy to access.
Here is my problem with the double, and to a lesser extent, with the single RS4, the camera hangs off to the side. It is difficult to do anything but stand or walk. Things like squatting down, sitting, or running, are problems. When I lean over to take a close up of a flower, the other camera will invariably flop around front. Also the camera's stick out from my sides and I have bumped them many times. In one case I hit something hard enough to crack the case on the back of the flash unit.
I bought the Cotton Carrier. It is very secure and does not have the swinging issues. However, it is not as comfortable to wear all day. During a long shoot, my chest was sore from the chest plate. Also the edges of the shoulder straps rubbed on my neck. I am going to try some more adjusting on the straps, but I don't think it will help beyond what I have already done.
In brief,
For one camera use; hands down Blackrapid RS4
For two cameras, no crowds, no leaning over, no sitting, not much moving around or just walking; Blackrapid double strap
For two cameras, active, moving around leaning over, sitting, laying on the ground, Cotton Carrier, but it is less comfortable that than the double.
And all three are a pain when switching to tripod mounts.