Paid for ultrasonic cleaning on an old shotgun I received from my grandfather. My FFL offers it as a service. It worked well on the old shotgun that was filthy although I am sure he had to use some effort the old fashioned way after the gunk was loosened by the ultrasonic cleaning. That's the only time I have used it since I do regular cleanings on my firearms and they don't get dirty enough for me to feel the ultrasonic is worth the money.
I have an ultrasonic cleaner and absolutely love it. For cleaning evil, black guns, and suppressors, it's a big, time saver. Perfect for a Glock slide, but I wouldn't throw a Colt Python in there.
Prior to my retirement I used ultrasonic cleaners for many cleaning tasks not related to firearms. However comparing ultrasonic cleaning to conventional cleaning is like asking if a dishwasher is better than manual cleaning. The reason is that many different soaps and or cleaners and additives can be added to a dishwasher, just as many types of solvents and even abrasives can be used in an ultrasonic tank. Ultrasonic cleaning is great for cleaning assemblies like a modular trigger assembly, complete bolt assembly etc. as it works by producing very tiny bubbles in the cleaning solution by very rapid vibration. These "bubbles" reach hidden areas and loosen derbies and in conjunction with the solvents remove that contamination .
I personally don't see much advantage to using it to clean the bore of a barrel or other readily accessed surfaces. Dependent on the cleaning solution used the cleaning can be safe or can be one that damages a surface IE: if an abrasive is added to the solution it can remove material, bluing, erode material edges etc. It works great for cleaning used brass, as it will clean areas not easily accessible by manual methods or tumbling or even vibratory cleaning..