Starkeeper...
My experience has been with Digitec and several of the onboard Yamaha VH's. And in my opinion, in no way do any of them duplicate the harmonies of live singers. BUT, they do add a little spice for a solo performance if not overdone.

Most any VH will offer a variety of harmonies and novelty voices which may be fun to experiment with. But, the real bread and butter value is in the flavoring digitized harmonies provide. It gives us solo guys a crack at making our performance litte more full and interesting.
Eddie