I looked at them a while back and a mate tried one. They seem to suffer from two "flaws" from my point of view. One is if you have a long lens on and kneel down the camera can unclip itself if the lens touches the floor. The second and more worrying is if it's quick and easy to unclip, it's quick and easy to steal.
One other factor I wasn't totally happy about is most use the tripod mount, this is fine except not all tripod mounts are equal, I've seen one ripped out of the bottom of a camera. That "mount" was held to the camera chassis with three tiny grub screws, not molded into the chassis itself.
c