Does anybody have experience with ultrasonic cleaners and old electronic boards?
I have experience with ultrasonic cleaners, but for cleaning ammo brass rather than electronics...
Based on that, I think it should do quite well for removing gunk from a circuit board, and is a probably a bit safer for small SMT parts than scrubbing at 'em with a toothbrush. OTOH, it's not going to brighten up tarnished/corroded contacts, and it's not a quick process either. (Plus the machines are annoyingly noisy... not something you want to run in your kitchen.)
All in all, for just cleaning up a few old commodity computer parts, I'd say it's not really worth the investment when tap water and dish soap are so effective and cheap. Ultrasonic would be better suited for especially delicate parts, or things with lots of little crevices that are difficult to clean in a normal fashion.