Hailed at its introduction as a 'holy grail of recording,' by Recording magazine (and adopted worldwide as the largest-selling audio plug-in of all time), Auto-Tune corrects intonation and timing problems in vocals or solo instruments, without distortion or artifacts, while preserving all of the expressive nuance of the original performance - with audio quality so pristine that the only difference between what goes in and what comes out is the intonation and timing. All with a user-interface that is a model of clarity, speed and ease-of-use.
And now, with its new-from-the-ground-up time manipulation capabilities, Auto-Tune 7 represents the most substantial advancement in functionality since Auto-Tune's original introduction in 1997.
For most common pitch problems, Auto-Tune 7's Automatic Mode instantaneously detects the pitch of the input, identifies the closest pitch in a user-specified scale (including minor, major, chromatic and 26 historical and microtonal scales), and corrects the input pitch to match the scale pitch. A Retune Speed control lets you match the retune rate to virtually any performance style as well as create the iconic Auto-Tune Vocal Effect.
For meticulous pitch and time editing, Auto-Tune 7's Graphical Mode displays the performance's detected pitch envelope and allows you to modify pitch and timing with a variety of easy-to-use graphics tools. This mode gives complete control over the correction or modification of the most elaborate pitch and rhythmic gestures.
Auto-Tune is used daily by thousands of audio professionals around the world. Whether to save studio and editing time, ease the frustration of endless retakes, save that otherwise once-in-a-lifetime performance, or create the Auto-Tune Vocal Effect, Auto-Tune is the overwhelming tool of choice.

Originally Posted by hautbois16

While I have never used Autotune in Reaper, I know that pitch correction can be adjusted to have a PROFOUND effect on the sound of a track. I record a one-man quartet; and on one hymn I did a few months ago, I set my pitch correction to its maximum value. The result was very solid tuning of all four parts - a sound unlike anything I have heard before! In some ways, I like the sound; yet I know that it would be rather extreme if I were to use pitch correction set that tightly on all of my quartet recordings. Still, I admire that performance as in a way the epitome of one of my goals of the quartet which I call 'Crystal Control.' That particular aspect of what I am after is to create a vocal tone as if the pitches were controlled or keyed from an electronic keyboard. My college voice teacher would run me out of town if she heard that performance, and I would agree that I wouldn't want that on many of the songs I record; but at times I like it. Yes, I have no doubt that a good pitch-correction routine can DEFINITELY make a difference between the corrected and uncorrected sound. In the extreme maximum settting, a singer with even a moderately wide vibrato might produce something which could sound something like a trill. I suspect that you will notice a difference if you find the settings needed to get it. Of course, if you happen to sing with a very straight tone as I do and you sing extremely well in tune, maybe the pitch correction doesn't need to do much to your voice. The phase difference you notice might be occurring due to a slight time delay introduced by the correction function.