I think this is the best song of all of her works.
Although, i'm not a very big fan of hers, i do find "secret of my heart"to be quite interesting.
Personally, my favorite artist is Utada HIkaru. She has a much better voice and much more original, (but of course, not everyone agrees) and can sing better in english.
but generally, this single's not bad. also, if you really like Mai, then maybe you should check out "Tsumetai Umi" that one's not bad either.