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.
 

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. 

Man, but it's spensiv !

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!
 

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.

Oh absolutely MH...I wouldn't change a thing on The Kingsmen's version of Louie Louie ( to name a favorite) which is riddled with almost every imaginable recording and performance flaw. It's always a judgement call and one should always err on the side of groove and vibe IMHO.

Well, you might be able to now with Celemony's DNA tech. Split everything out, clean up the vocal and see for yourself once and for all that there were no racy lyrics:-)

That would be like colorizing Citizen Kane. Or would it?

I feel no shame in saying I use Melodyne frequently, and is one only just a few FX I use outside of the ones that come preinstalled with Sonar. Most of my recording concentrates on vocals and I will use whatever FX I can to make those vocal lines pop.
There are some musicians that feel pride in the fact they use no pitch correction software but have no trouble admitting to re-amping a guitar solo, adding reverb to a cold vocal line, or even overdubbing a fluffed note. I don't see any difference whatsoever.

As a home studio singer/songwriter/player i always end up buying tools like M editor then not using it...i find comping much more fun and sitting there tweaking stuff less exciting.

Or... I'm just vst addicted and ultimately lazy...idk...but i end up just using a small part of my rather huge arsenal...

M4studio looks amazing... I swore off vst purchases this year to learn what i have..but M4studio looks amazing...i can't repeat it enough and truly appreciate this thread...as i do need to begin spending time learning some of the tricks...

The tempo system...the ability to just free form play and sing...omg...thank you.

Watching the vids...I'm pretty floored... Technically this it's an upgrade...right? Right!

Thanks for the liberal buy in timeframe...it will allow me to recover from the huge number of amazing vsts i got last quarter...And it's hard to pay bills gardening during rainstorms!

I think M4s has just moved to the front end of my "really learn to use" list.

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.
 

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

 
After:

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.