Actually, it does look kind of like fun to me. It does address the fact that the upper body is mostly just along for the ride on most bike rides. I wouldn't use one as my Going Places And Doing Things bike, but the odd trip down the block or along the bike trail would do the core muscles good.
I'm not ready to buy, but if someone would give me one. . .
Give big space to the festive dog that make sport in the roadway. Avoid entanglement with your wheel spoke.
(Sign in Japan)
1978 Raleigh Gran Prix
2003 EZ Sport AX