Very, according to Shawn at King Kog. Apparently they basically force you to ride with perfect form (as any mashing/movement results in bounce), and slowing down while seated is nigh-impossible to do without getting jostled around.
I own a pretty similarly geared riser road bike; if you're used to big-ratio fixed there's plenty of gears even for relatively hilly city riding, and the ability to go like a freight train if the opportunity arises is stupid and unnecessary but fun.
Oh and
