A nice tool, even for the professional.
Okay, so it is designed for the "Auto-Tune/T-Pain" sound and it does this perfectly. Just as expected. If you want this sound, this is it!
However, there are other great reasons to use this plug-in. I own Melodyne Editor, AutoTune Evo, Auto Tune EFX and Waves Tune and they each serve a purpose.
I find myself using this plug-in (although version1, I don't own EFX2) when I am stacking tons of back-up vocals that need some pitch correction (using the gentler Pitch Correction setting). I choose this plug-in over all others for 2 reasons.
1. It is very light on the CPU. I can get way more Auto-Tune EFX plug-ins than I can get Auto-Tune Evo's in a project. So if I am stacking 20 vocal tracks (which has been the case a lot for me lately), I can throw this on a ton of tracks.
2. If your DAW allows you to do automation across multiple tracks at once (such as Pro Tools), then you can pitch correct multiple tracks at once by enabling automation on the individual notes across all tracks and then editing the automation on one of them. So lets say you are in the key of F and there is a point where the singer continuosly was singing F flat, so auto-tune is thinking it is suppose to be an E on a bunch of them, then you can turn off "E" for that moment across mutliple instances at once.
The new "Auto-Motion" feature in EFX 2 looks and sounds silly to me, but I don't have version 2 to really comment.