Rode one around in the parking lot last year at the Southeast bike expo and thought it was a very cool.
Last month was down in northern Florida for a 50 mile event that included a lot a deep sand. The guys on the fat tire bikes just floated over the sand like it was nothing while the rest of looked (and felt) like we were riding through quick sand.
It's not a bike that I would want to ride on my local trails (it's just not a good fit), but for the right environment it will be a blast.
Cycling... If you're not having fun, you're doing it wrong.