Actually, I have one of each - a speedster and a terra-shark... bought them loooooong ago... like, 10 years ago long ago... they're both very fun, rare, interesting bikes... sadly, I never had a chance to build my speedster as a belt drive. Should be a rohloff, though!
Can't say that I'm a fan of the way this guy has built these up, though. They just look... a bit... odd?
As for how they ride - the speedster is realllly only for piddling around on roads... it is not strong enough for the likes of even a gravelly towpath, IMHO. It looks bad-a$$ though when built as a skinny tired, drop bar speed machine!
The terra shark; it's massively stiff and the finish is amazing... but it (along with the speedster) takes a freaky weird seatpost size which took me aaaaaages to find.
I think they would be "buys" at around 600...