I have the basic RS-4 and love it. ; Once I used it with a 70-200; screwed it into the tripod socket on the lens and it balanced perfectly.
I tried using the pocket on the strap to hold memory cards, but found that they sit on my collar bone and cause a little discomfort.
At Pixelmania I had it on my battery grip and found that the standard FastenR-3 got in the way under my hand in portrait mode. ; I just purchased a FastenR-3 to see if it feels any better.
One minor complaint is that the strap tends to move off my shoulder and down my back occasionally when I raise the camera, so I purchased a BRAD strap to keep the whole thing in place. ; I'll let you know how it works out.
I also have the FastenR-T1 mounted on a Manfrotto quick-release plate. ; I haven't had a chance to use it enough to give an opinion.