I've been using a TC Helicon Harmony-G for several years and ABSOLUTELY love it. It's now been replaced in the product line by the Harmony-GXT. How natural it sounds is mostly a function of the user's selection of effects and how you place them in your material.
We do some songs where I turn it on for 1 word in the song. In others, I use it for the entire song. And, of course, in many songs, I don't use it al all.
The demo videos I've watched are designed to illustrate the device's capabilities. In actual live show use, it's a bit easier to create the illusion; however, the audience is still faced with hearing more singers than there are people on the stage.
IMHO, doubling, properly used, should be virtually undetectable by the audience. They should just think the singer's voice sounds full and rich.