Yeah, I do these regular "eclectic" shows where I cover everything from SInatra to Sabbath, and I recently added a TC-Helicon Harmony-G to my live setup. Obviously not the same box, but the same idea.

The "authenticity" factor jumped pretty dramatically, and the audience REALLY noticed. I get comments all the time about how much the harmonies add to the show.

You're right about using it judiciously. I find the most satisfying uses are tasteful and kind of unexpected. Like when I cover "People Get Ready", I'll kick on the harmonizer for just a line or two here and there, and it adds a sweet little taste without overshadowing the more emotional "solo" vocal performance during the rest of the song.

Of course, songs like "Don't Stop Believin'" and "Wheel in the Sky" NEED the harmonies or they just don't sound right.