I have an ultrasonic cleaner and it really did a number on my sig mosquito, took some of the finish off of the slide as well as the barrel. I've also put some nickel plated parts in there and it ate some of the nickel away down to the copper plating (probably just a poor quality plating job). However...it worked wonders on my stock DPMS AR15 bolt and bolt carrier. It didn't touch the finish on that and cleaned it better than I could have with anything else.
As long as the part has a good quality finish/plating and you're not using an ultrasonic cleaning fluid that can harm the metal/metal finish(such as sodium hydroxide on aluminum) it'll work great. It does wonders for cleaning brass, although it won't remove any permanent staining so your brass will stay dull colored.
One last thing, make sure you remove all of the water/cleaning fluid off of your parts as soon as possible to keep them from rusting. Your parts will come out without any protective oil left and will rust that same day unless all the water is removed. Then oil them well to prevent any rust in the long run.
For the record, I'm using Blue Wave ultrasonic detergent that I used through my workplace. A mix of both 25-I as well as Dril-Lum 603 works wonders on just about anything, but the company requires a large minimum order so it's probably not something that many people would come across.
http://www.bluewaveinc.com/b6.htm