I have used an ultrasonic cleaner extensively in repairing/restoring older high-end microscopes as well as fine fountain pens made from the 1920s on. Lots of good points have been made here, so let me try
to summarize and a few of my own:
-ultrasonicate individual parts, not assemblies, unless the assemblies are tight. If parts rub against one another, wear will be greatly accelerated.
-Dawn and water clean well; use a very dilute solution.
-Always run your cleaning solution for 5 - 10 minutes before adding parts. This degasses the water and greatly enhances the cleaning action.
-if you use a safe solvent, put a small quatity in a little jar, and place that in the water in the sonicator. Clean parts in in that jar. Obviously, this only works for small parts!
-Alternating between utrasonication and brushing to clean works well.
-Warm cleaning solution always works better.
-Keep your fingers out of the solution when the machine is running!
Finally, don't overdo it ultrasonication takes minutes, not hours! Don't use strong clearers, rinse well, and protect the cleaned part against rust. If a part is only lightly soiled and you can reach all sufaces, just use a brush. Ultrasonication works best at getting into crevices.