I've seen quite a few around here. I find them a slight irritant because I can't stop myself chasing them down (my problem, not theirs!). There's one in particular which I often come across on my commute home.
That said, I do quite like the idea, and we've toyed with the idea of getting one for my wife to help her tow a child trailer up some of the hills (It might mean she could use the bike more for general shopping... We live about 7 miles and 1,000 feet from the nearest big supermarket).
I'd really like to borrow one for a month or so and see if they're up for the job.