I used Essential in Sonar X3 Pro twice to play with it.  but it seemed so limited that I haven't really used it since.
 
v4 studio looks like a lot of really cool features but I probably wouldn't really use it.  but GAS doesn't prevent me from lusting after it haha.

I use it often but I am not running a commercial studio. 
It is not bad in any way to have to use a tool to correct a live take for any reason.
I am a good singer but there are plenty of times I might have missed that high note at the top of my range. 
I fix it.
Other times, I may be in tune but I rushed it or came in a touch too late but I like my delivery. So I fix it.
I also like to manually adjust the volume of a word or phrase by editing the blobs. It's very easy in Melodyne.
 
Now I am totally interested in how Studio 4 does auto tempo generation. This is something I have longed for for decades. 
 
I would be very unhappy not to have Melodyne in my tool kit. 

Yes sir, me sir.  Along with most every other producer on planet Earth.  And please don't kid yourself.  Melodyne and Autotune are used on even the best of singers if only for minor tweaks.  It is used just like you would use a compressor or an eq or any other processor...to enhance the recorded performance.  If you are not using it or its equivalent on your singer you're shortchanging them.  Why not just stick an SM57 in front of the artist(s) and record every instrument and vocal directly to a wax cylinder?  Sheesh.
 

Ol Pal is right...

It makes it easier for the vocalist to relax to know that they CAN miss and still
be ok...and reduces the number of takes the Recording Engineer has to capture
and comp...

Granted, some singers just "have it", but, when making an album, and trying to keep
the costs to minimum, this is, by far, the best tool for the job...and only logical to use
it...even it you don't use it to make any adjustments...at least you can "see" what the singer
did, and where the notes landed...

But, hey, that's just me...it's a tool, like anything else...and you can either use it or not...
that's your choice...

One use for editor not yet mentioned is converting to midi. Sometimes I have musicians over who record originals and I want to add tracks later that I play.

I convert the performance to midi. Then, I can easily change tempo without changing pitch, learn the part, then resume the original tempo and add my tracks.

I'm planning on the upgrade. I agree with the op that I don't get a lot of use out of it. And when I use it, I have to relearn how it works.

One of my constraints is always isolation. You can't pitch correct a guitar tone that is also going in the drum Mics.

Sure is tempting to be able to fix the same pitch across a set of tracks. And I feel no shame in making a great recording people like listening to by using a tool like Melodyne. Frankly I'm way more interested in the synth like controls in version 4. I love to distort and make weird sounds.

Use it, yes. Indispensable, no.
 
My usage has steadily decreased since I first got it back in 2006. At first I went overboard - and have some embarrassing Kanye-esque yodeling to remind me of my past excesses. But for the last 4 or 5 years, I've used it only rarely, typically to correct a previously unnoticed error after the fact.

If you EVER heard a raw vocal of mine you'd know why I use Melodyne. 
 
IIRC I've only released one track of mine where I not only did the vocal from top to bottom but I did not need to pitch correct anything. It was a perfect storm for me.

+1 to most of the above.   I've used it many times for lead and background vocal pitch correction and minor timing adjustment, for minor harmony changes when a note doesn't seem right after all, for dragging audio to MIDI tracks for synth doubling/reinforcement, and on occasion for purely creative purposes (something I should do more ).
 
Yeah, I'm a Melodyne fan.     Good thing the intro offer on v4 lasts until April. 
 

I'll be all over the E2->S4 upgrade next week. I'm chomping at the bit.

I use it almost all the time.  Everybody has a moment where they miss a note.  But, for me, the strengths, other than pitch correction, are the ability to change the timing of a word, as well as stretch a single note.  I think it's an amazing piece of software.