This is exactly what prompted us making this thing---some of my co-workers at oDesk were talking about how dissatisfied we are with the whole 1-10/NPS score way of getting feedback.

As far as the slider---my motivation was that humans are sensitive to the emotional content of even very tiny changes in a human face (which is the logic behind the Chernoff face http://en.wikipedia.org/wiki/Chernoff_face) and that the smiley-faced slider would get people to (a) use the "same" scale for conveying their feelings and (b) convey their feelings more precisely. It's on our agenda to do some tests with it---shoot me an email if you want to try it on your site & see how it does. It probably won't make it into our product too soon :(