I find that if you use it on single vocal lines you are more likely to hear the robot-y effects than if you use it on blocks of harmonies, so I just tend to use it on chorus-type vocals now. Its best if you spend a while trying to get them as good as possible, then use melodyne subtly on top (you can set the "amount" of pitch correction, so play around with that to get a good balance between perfect pitch and robot voices)