We used them on everything from eyeglasses to jewelry to surgical impliments (before the went in the autoclave!) They'd work great - until they broke. Usually no warning either. Yeah, you'll find cheap stuff on ebay all the time - it's a dime a dozen to coin a phrase. You *might* get lucky and get a hold of one that lasts for a few weeks or months....and you might get one that stops dead after two 30 second cycles. I've seen both ends of the spectrum.
The thing is, an ultrasonic is good for SO many cleaning tasks. Metal watch bands are a cinch, musical instrument mouthpieces like trumpets etc., paintbrushes, all sorts of stuff. So it'd be a great investment for the household. Make sure the wife/girlfriend/both know about it's jewelry cleaning capabilities too - they might even offer to buy it for you then!
"I may not fly with the eagles.....but weasels don't get sucked into jet engines!"