Wow, that's a good question!
I remember a long time ago, before the original Kingdom Hearts came out, an advert came on TV for it. I was obsessed with the song, and wanted forced my friend to download it for me. I never really put much thought into who sung it, because back then it was put onto an MP3 player without any details (I don't think it even had her name on the song).
When I got my first iPod, that's when I became obsessed with getting every piece of info. right on songs, filling in all the blanks (I know, I'm weird :D).
And when it came to Simple and Clean, I had no idea. So I researched it on Wikipedia, and found out the singer was actually Japanese-American. I was like "Wow...".
And then I saw she had an entire back-catalogue of songs. However, this was much later, in around 2004, after the release of EXODUS. I honestly cannot remember when I became obsessed with having everything Utada's ever done.
I do, however, remember that I liked Megumi Hayashibara as well, and wanted to have more Hayashibara songs than Utada, so for every Utada song I got, I got another Hayashibara song.
Today, my Utada song collection stands at 308. Megumi still stands at 25, after I stopped caring, haha!