I wasn't that thrilled with "Under Your Spell" either. It seems to have been a personal favorite of JOss Whedon himself, at least from his comments that I've heard. I don't know, I have to tread carefully here but I thought the song was sort of unfinished. And it may be a love piece, but when you know the story the theme of the song darkened by the fact that Tara is literally under Willow's spell when she sings it.
I was impressed with Amber Benson's voice in the other numbers, her duet with Anthony Stewart Head, and the aforementioned "Walk Through the Fire" ensemble. And then in the closing number "Where Do We Go From Here" I believe it's AB singing the high pitched harmony. Her voice has an impressive range.