They come in several flavours. but the 2 most important things to look for are the power rating and weather or not the unit has a heater.
The principle is actually very simple. a high powered ultrasonic transducer in a tank of water causes cavitation bubbles that collapse on contact with the item being cleaned. The results should be a fantastic mechanical cleaning action that gets into all the nooks and crannies that the knackered old toothbrush cant.
I saved up and got mine from Cookson Gold. Along with a bottle of cleaner. At the time I was also making jewellery and this unit was a big investment for me.
Unfortunately, the results didn't live up to the hype. I was very disappointed with it. I did extensive reading to try and figure out what I was doing wrong. but the bottom line was that nothing this unit was doing was any better than a few seconds under the tap with washing up liquid and an old toothbrush.
To start with, this thing is very unpleasant to be near when its running. I tended to pop out for 10 minutes when its running.
I played with it for about a year before finally relegating it to the cupboard. I would love to hear from anyone with any suggestions.
Things I have tried range from different chemicals (Ammonia solutions, white vinegar, water, soap solutions, posh stuff from Cooksons...) and different temperatures with all the above.
My advice... don't buy one.