I'm always on here as the heretic who actually thinks Auto-Tune (in graphical - not auto - mode) does a better job tuning vocals with fewer artifacts than Melodyne. I still think that.
However, I recently used Melodyne to change a single note in an arppegiated guitar chord. The result was a chromatic run that really added impact to the outro of the tune I was working on.
Glad I have both installed and can use whichever one does the better job on the task at hand.