Hey,
there have been many questions about this unit in the past. It looks like most people are not answering yet, so you might try searching for TC HELICON on the site.
I have used it, it's ok. I would like to see the newer VoiceOne and whatever else they just came out with.
VoicePrizm -- The sounds are good as backup tracks way down in the mix. The FX are loud and the unit it somewhat hissy. The harmonies are quite nice and tons of fun to play with though. Nothing beats being able to actually sing harmonies, but since I can't very well, it is a cool unit. The Plus upgrade is a must from what I have read.
I don't own one, but have used them a number of occasions. They are alright. You might check out the other stuff out there that does harmony though....