My experience of ultrasonics, having used them for 13 years:
A good-quality ultrasonic tank will last a long time. My 3-litre Elma tank was the first one I bought, 13 years ago. It gets used, on average, twice a week, for at least an hour each time. It still works perfectly, and is in very good condition, being all-stainless construction. This tank is used for cleaning clocks, and is left filled with Horolene fluid 24/7. The downside with this fairly large tank is that it takes about 20 minutes to heat up.
3 years ago, I bought a small plastic tank, similar to the on pictured above. This one was used, until very recently, for cleaning watch parts, very small clocks, slot cars, and RC electric motors. The tank is partially filled with water, and the parts and cleaning fluid are put into small plastic pots (spray can lids), which sit in the tank. This keeps the tank clean, and doesn't seem to affect the cleaning process. For movement parts, I use L&R fluids. For watch cases and slot cars/RC motors, I use tap water with a dash of Fairy dishwashing soap.
This year, I bought a larger plastic tank (1 litre stainless liner) with a more powerful transducer. This cleans faster than the small one, and has the benefit of holding more plastic pots. It also warms the fluid, which aids cleaning. This machine is getting a lot of use, and has performed flawlessly.
For occasional use, the £20 eBay machines are fine. For the hobbyist, it's worth spending £60 for a larger, more powerful, machine.
For movement parts, an ultrasonic will clean using safe fluids. You can use manual methods, but only if you are willing to use some pretty nasty solvents.
