I know a couple of guys that use it and they love it. But it seems they are expensive. The people I know make their own out of innertubes. They go to a tractor store and buy the material. I beleive there is a youtube video that shows how to make one and use one. I would check but I'm having trouble with youtube for some reason.
Another twist on the kettlebell as I see it....
i seem to remember a bulgarian gentleman mentioning this , but he may very well have been referring to his wife.