Never used one, always thought it would be cool for cleaning chains...until I read a thread about it recently on velocipedesalon.com forum where someone pointed out that ultrasonic cleaners remove too much of the lube that you want to stay deep within the chain links, and effectively re-lubing the chain after such a thorough cleaning would wind up being more trouble than the cleaner saves. Not sure that would stop me, but it did give me pause.
For other parts, still seems like a cool idea.
My local wrench has a soak tank for cleaning parts -- not sure if it's ultrasonic, might just be static -- but he swears by the new "aircraft-grade" Simple Green for filling it.