I actually love the song. The piano parts are just gorgeous. But... it's her voice that almost ruins it. Like others have said, her voice is nasally, but in addition, she just... sings with no emotion at all. The "aitai yo" is repeated so many times that it can REALLY become an annoying part of the song if it isn't sung right, with a little variation... but she just sounds like she's bored and practicing this song or something.
It's a shame, really. I'd kill to hear someone else singing this (particularly Eri. I think this would suit her voice well for some reason).