As I know T Pain, I officially met him back stage a few years back in Atlanta at a Tech N9ne show, and I know Aaron Connor (the 1st engineer to use Auto Tune for Sher's Believe in Love), Aaron he told me the whole Cher story one night years ago while we were working, I feel free to comment here:
Aaron used the box after seeing it at Namm, he mixed believe then added the Auto Tune box and surprised the producer and the rest is history. He used it as an effect for a new cool sound.
T Pain redefined how an artist uses Auto Tune, he was the first to actually record with the TDM plug on while singing and not after. T Pain perfected singing vibratoless with Auto on while recording the song up front, in this he changed the way every Atlanta artist recorded, which spread everywhere.