It's a very powerful tool.
don't be fooled by thinking it's "just for vocal".
I, like others have said, have used it on lots of stuff.
 
Had the Editor for a while, but recently upgraded to Melodyne 4 Studio.
 

Indispensable.
 
The best thing about Melodyne is that it can be used in such a way that no one knows you are using it; it can be gently used for a natural sound.
OTOH, It can be adjusted to sound like "Auto-Tune", if you really want that chromatic sound.
 
I use it mildly on a singer so it doesn't sound "auto-tuned". It can really improve the vocal track.
Some singers use way too much vibrato on a few notes. I can reduce the vibrato on select notes.
 
Used on bass guitar. Sometimes a cheap (or poorly set up)  instrument will have intonation problems. Melodyne cures that.
 
Used to match the tuning of various instruments to an acoustic piano. Every once in a while I'll get an acoustic guitar track from someone that used their tuner instead of tuning to the AC piano. Select all notes and slide the tuning up or down as necessary. Cures the tuning with no artifacts. No need to tell the guitar player to redo his track.
 
Lead guitar: The guitarist can play a killer solo but sometimes the bends don't reach the final note. Fixed with Melodyne. Ditto violin.
 
Drums: create a midi drum track for kick and snare. Mix these midi samples with the real drum to make it bigger/better/fatter.
 
Melodyne also used to stretch a vocal "ahh". Singer sang the part too short: Stretched with Melodyne.
 
Timing issues with bass guitar, some notes too long, some notes too short: Melodyne to the rescue. No need for bass player to punch-in or redo parts.
 
 

I don't know if it's indispensable...but it sure comes in handy at the right times.  Having the right tool for the right job, so to speak. 

You can lengthen/shorten/move/change pitch/detect tempo/adjust the material down to it's core for God's sake - who wouldn't want it AND use it?!?

Man, but it's spensiv !

I'm producing my own songs and the music is mostly in the box, but I record my own vocals on every song.  I've recorded other singers a few times.
 
I'm usually on pitch, or close to it, but I've used V-Vocal and now Melodyne to make small adjustments.  In Melodyne I'll select a key and manually tune with the sliders set near 15%.  At one time I thought I'd used Melodyne to create harmonies, but more and more I just sing those too.  I did use V-Vocal to adjust consonants, breath noise, volumes and such on vocal clips, but now I mostly edit/automate with Sonar's envelopes.
 
I will use Nectar 2 for harmony, when I get lazy and the song can handle those kind of background vocals.
 
This tune contains of all of the above:
 
https://www.reverbnation....ong/25055688-the-storm
 

It makes me wonder how all those classic hits were recorded in the "old days" before all this correction technology was invented? It's funny now to listen to the oldies and classic rock stations and hear all the performances that make you wonder, "Why didn't they fix the intonation or adjust the length of that of that phrase?" or "Geez! It seems like they could have at least told Hendrix, Clapton, and Page that their guitars were a little out of tune!!" How about, "Gosh! Mr. Dylan! That vocal is really all over the place pitch-wise. Don't worry. I can fix it with Auto-Talent!!"
This is all said in jest!!! I do not mean to poke fun at anyone. I just am concerned that sheer emotional and well practiced TALENT is falling by the wayside. I am all for any technology that helps us in our day to day struggle to get the best performance we can but I think sometimes we all get too obsessive in our hunt to correct EVERYTHING!
A drummer I worked with for many, many years looked at me, after seeing the first drum machine in a studio and said, "Well, Sid! I guess it's only a matter of time before they'll figure out how to put guitar players in a box and we'll both be out of work." BTW, he still plays his butt off sitting behind a set of Roland V-Drums!!!!
Don't worry. I upgraded my Melodyne 2 Essential to 4 Essential. And I will probably figure someway to afford the jump to Studio. The only thing that bothers me is since 2 came in Sonar I have only used it once on a couple of bass notes. Boy, I need to see a doctor about this GAS!!!! LOL!
 

Well said, Sidroe!
 
I once watched a late-night infomercial for classic C&W hits that featured old live TV performances. Had I not known their vintage, I would have assumed they'd been tuned. But such things were not an option in the 50's, 60's and 70's. Those guys were just singing on such good pitch that they sounded as accurate as Britney Spears. (And live-mixing their own foreground/background/harmony balances in real time by adjusting the mike distance as they sang.)
 
But I noticed another telling observation: few of them had the good looks of a model. Few of them would be successful today, I'm afraid.
 
I blame it on the industry's desire to duplicate performers like Elvis Presley, Rickie Nelson and Pat Boone - performers who both looked good and could sing. Once it became possible to make anybody sound "good", you could select the next superstar by looks alone.
 
One of the earliest of those, AFAIK, was Billy J Kramer, a handsome guy who couldn't carry a tune in a bucket. He had success, though, via multiple takes, lots of editing, professional singers subbing parts, a very good band, and perhaps most important, a couple catchy Lennon/McCartney tunes that had been rejected as not good enough for the Beatles. Mr. Kramer was very successful with record sales, but tellingly did not tour much.
 
Here he is, obviously lip-synching (he didn't even do that well):
 

 
 
 

Actually there was pitch correction applied not infrequently before any digital hard or software was available. It was done by subtly controlling the speed of the tape playback on one machine and recording it on another. I remember a session we did where a relatively famous singer was hired to sing a jingle package for a national heat and air company. She was a joy to work with (and still performs today) but she was dreadfully flat. We used the vari-speed on our mastering deck to...painfully slowly and requiring many tries...pitch her up on the worst notes. When I listen to those tracks today I can still hear she is off. But for most folks it just passes by unnoticed.

We would've killed for Melodyne back then.

There are whole websites dedicated to locating all of the mistakes that were on Beatles albums. I love all of those warts, every one of them. In today's music business, they wouldn't be put out that way. In today's Song Forums, those items would be flushed out in a second and be the topic of the moment. It's seems that is the new norm for what is expected. I wish it would swing back the other direction some. I tend to leave some of the imperfections in my songs, but I am often told they need correction.