Well, I've been using the single-pulley Campagnolo "Sport" rear mech for a week now. So far, not bad.
The chain has slipped over the freewheel only once since I moved everything further back in the dropouts. Just that little extra bit of chain tension has helped, but I still worry when I stand on a hill or accelerate from a stoplight - smooth and steady pedaling seems to work best.
As far as shifting is concerned, no problem there. It shifts as well as the contemporary Gran Sport. There's extra cable friction from the full-length housing, but not enough to be a problem.
I'm not quite certain why these have such a bad reputation. I wouldn't want to race with one, but if I can manage it while carrying my 190 lbs around a hilly city. . .
Next stop - 1948 Simplex "Tour de France" plunger-style double-pulley "single bend" 5 speed rear derailleur :-)
Aldo Ross "any derailleur will shift if you're patient enough" Middletown, Ohio