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 :(
I would very much love to hear on HN about any results that you or anybody else come up with. My suspicion is that if this works out as you intend the distribution won't even require any fancy math to show people use it differently, it'll be blindingly obvious in a histogram. It's brilliant.
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 :(