I always enjoy these type of discussions.   My primary period of "musical growth" was the late 60's through late 70's, so I can identify with every comment above regarding the vocalists of that era and what they were able to do.   Unfortunately my vocal talents exist on an entirely different planet so I'm glad Melodyne exists!    I do attempt what Vastman mentions by shooting for the best takes possible, but as Tim also mentioned there's a limit to that... and the point of diminishing returns happens all too fast.    Did I mention I'm glad that Melodyne exists?
 
Given the use I've gotten out of Editor v2 I am so looking forward to Studio v4.
 

Those thinking , " the singers i work with don't  need pitch correction " may be decieving themselves.  Very few people have 100% perfect pitch 100% of thd time,  particularly during live perfomences where each of accompanying instruments may drift in and out of tune. A friend of mine once dug out acapellas of noted singers to illustrate that even the greats occasionally drifted. Fortunately the human ear is fairly tolerant of imperfection. I suspect melodyne is used far more for effects, harmonies,  artistic decisions to change pitch more than  to try to rescue someone that cannot sing at all. Even in pop music where people who cannot sing make hits with autotund, there is no pretense that person can sing. People like the effect for some reason. Future is selling buttloads of records and even his staunchest supporters would not argue he could sing a lick. 
 
I am fortunate that nobody I deal with desires that sound or even is under the delusion  i can make a non-singer sound good. They do love some of the crestive things that can be done.

There are those who'd have no career without pitch manipulation.
 
Before:

 
After:

I have only used it for pitch and timing correction once, on a guitar part. I've used it for pitch alteration, i.e. changing the melody of a vocal sample. 
 
However where I love to use it is it's audio to MIDI conversion. I use that ALL the time. For instance I recently wanted to use a keyboard riff from Native Instruments' George Duke library but with a more contemporary synth sound. The polyphonic mode did a great job of creating the MIDI for me. But I don't always use it for such straightforward conversion tasks. One thing I love doing to get the creative juices flowing is to take an audio sample - anything from a solo instrument part to an ensemble or band - and running it through Melodyne polyphonic to see what it comes up with MIDI wise. If it's a busy part with multiple parts, what usually comes out is somewhat different than the source material because of indistinct notes and harmonics. But that's what I love. It gives me some raw material to chip away at like a sculptor. It's a totally different approach to starting with a blank piano roll. I delete notes that I don't like, move others around and maybe even add a couple of my own. I don't consider it cheating because I'm using my musical judgment to "sculpt" it, and what I end up with is usually something wonderfully quirky and colorful that I would have probably not come up with without Melodyne. I love it!

dubdisciple
Fortunately the human ear is fairly tolerant of imperfection.

I would go further and suggest that the human ear, like the human eye, likes imperfection. 
 
Having said that, it's not an exclusive like. It can also appreciate perfection too. Sometimes there is nothing like the clinical perfection of a drum machine in the right context. It appeals to a different part of the brain.