I don't own one and I didn't get it at first, but after riding one I definitely saw some of the appeal. They are very fun to ride in a "I'm totally connected to this bike" sort of way. If you also take off the hand brake, then there's an added element of fun (and terror) because you're using the same mechanism to go and to stop. There's a sort of minimalist joy in the experience.

Really, if you get a chance, ride one for a bit. They're not super practical on hills, but they work surprisingly well because they're so light.