I tried a couple and they're a blast, I had a hard time on the entry level bike, it was a bit tight for my height, but the Blata was doable. I'm still somewhat interested in them only because I have a couple of go-cart tracks that do track days for those things, and 1 of them even has a race day.
The high-end pocket bikes can "supposedly" do 80mph, but I just can't see a 50cc engine pushing my 200lb butt that fast.