Do any of you serious producers use this plug-in? I've seriously been thinking of buying it since it's an industry standard. The vocalists I usually work with are gigging musicians or students at the local university, but I still have problems keeping them in real tune considering I also do a lot of back up harmonies.

Worth the investment?
I was once heavily prominent on these forums from 2004-2007, let's see how long I can stay now that I'm back.
It depends on how much time you're willing to spend on it. The automatic mode is really not worth the bucks unless the changes are miniscule and you tweak some parameters (a lot). Manual mode is excellent though but it is time-consuming and as before, you need to be fairly close to what you're after to get good results. Overall I'd say it's not worth the investment, for a production to sound really good you need a good vocal performance (a composite most likely) to begin with that you can then tweak in minute ways to make for a near-perfect recording. Those kind of small adjustments are better done with a good pitch/time correction software like Serrato which you can also use on other things (Auto Tune really only works on monophonic sources in my experience). I haven't had time to check out Melodyne yet but I hear good things about it as far as transparency and easy interface goes.
IMO it usually sounds bad and i can always tell when its been used. Just do multiple takes and do a composite if you have too.
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Thought ebon00 gave you the skinny on the technical aspect, I thought it was worth noting that serious producers use Autotune - all the time. True, it's detectable when using in access, but it can improve a good singer in a subtle way.

Though I'm assuming you're a moral person, one could download a pirated copy of the program for free as a bittorrent. I'm NOT saying you should or saying that I got the Waves Diamond plugin that way...just merely suggesting.