Much of the above information is right on the money. The vast majority of onboard harmonizers provide marginal harmony quality at best and must be fine tuned to achieve this. That's why I swithced to the Digitech VR when it first came out and never looked back. Unfortunately, this processor is no longer made.
One of the problems with performers using vocal harmonizers is overkill. They tend to use it as a lead voice instead of harmony accompaniament, which is the use for it is intended and designed for. And, as DNJ stated, it's an artform in itself. You cannot expect to sing, hit a switch and have beautiful harmony mysteriously appear--it just don't happen that way. You need to set the system up for you, fine tune it to the Nth degree, save the settings, and practice using lots of vocal control. The trigger mechanism is quite complex, sensitive to tiny quirks, and without voice control it's easy to end up with lousy results.
Vocal harmony processors can, and often do, add a lot to the routine of a good performer. I've heard UD live using his Digitech VR years ago and the results were amazing. Of course, his right hand chording is amazing as well. (I didn't know humans had that many fingers on one hand!
) Throughout his performance the vocals were crisp, clear and distinct, and harmony was just a small part of his overall performance.
The bottom line: There is no magic bullet, magic black box, or magic anything else to create quality vocal harmonies. Instead, it's the combination of setting up your keyboard's vocal processor, plus individual singing and playing ability. Eliminate any one of those components and the best equipment in the world won't make a bit of difference.
------------------ Travlin' Easy