Melodyne Editor (ME) is an amazing tool. I've been using ME for some time now. Used properly, it's an essential part of my workflow.
I agree... the closer to "right" the note is to start, the better ME will do in correcting it. In fact, with most notes it is totally transparent and leaves NO artifacts in it's wake. I have used it to create a female harmony from a female lead/unison melody line and it was pretty freaking amazing. ( First female harmony part in my song "Coming Home" on my music page)
I have used it in poly mode a few times. Mostly I do try to avoid needing it in that mode. I used it to remove some wrong notes from a chord in a wave sent to me by another musician......it worked fine. Efforts to stretch notes in poly or change notes more than a step are not as successful as I would have liked. I often resort to plan B when that occurs, whatever that might happen to be at the moment. I have used it a few times to fix chords and it does a respectable job, as Anderton mentioned, of being able to fix that errant note in the chord without screwing everything else up. It can easily move the entire chord as well and the listener will not be able to detect that it wasn't right to begin with. I used it to fix some mandolin on a song and it worked well.
When I do my pitch correction, I do not let ME run in the autocorrect mode. I have been using the manual mode for some time and tend to like it better. I make the call on a note, not the algorithm. When I finish with a section I save the project and move to the next section taking it phrase by phrase mostly using ME on vocals. Once the track is finished..... I simply use the APPLY AUDIO FX function and ME gets printed to the track and the plug is removed from the bin. I don't let ME set in a bin doing the processing, I convert it to audio. My reasoning is that I don't need the out of tune vocal track once it's been fixed by ME.
ME is much deeper that what I use it for and has capabilities that I haven't even explored much beyond watching the tutorial videos. If I have a need for those functions, I can watch the video and experiment.
All in all, ME is a tool that I would not want to be without.