I have a set of U's on my Bianchi Migale's rear. The brake pads are stock from 1990-ish, and the stopping power is still more than acceptable. Haven't really tested it in the wet though.
Pita to set it up, limits clearance and looks crap though, I even prefer cantis to them...
Thanks. I'm surprised how well the U-Brake performs (under dry conditions...).