I've never had one, but they have been around for a long time. I remember reading reviews back when I was doing a lot of off-road riding back in the 70's.
They just did a segment on them on TechTV, and there's still a loyal following, they have a big annual rally. As I recall, they were quite slow, being geared very low, and have big, fat "implement" tires that can float over mud and snow. Used to be they would float!