I tried one at a trade show the other day. It seemed kinda fun, but I wonder if there is really any benfit to it. I see they make them for horses as well. My little JRT seemed to really enjoy standing on it as well.
A friend of mine has one, uses it regularly and has for at least 2-3 years, and got a good report from her doctor about how well her broken heel had healed .. and that in spite of her tendency towards osteoporosis.
(suggestion ... do what you can to avoid breaking your heel ... she was short-cutting through a parking lot, crossing through some vegetation and stepped/fell into an open irrigation control vault, landing full force straight down on a pipe.)
eta ... like any exercise device, you have to actually use it.