I don't really like Ai Otsuka's cutesy upbeat stuff that much, but I do think she can sing really beatiful ballads. Renai Shashin is one of the best ballads of 2007, if not THE best. Daisukidayo , Kingkyo Hanabi are also nice
Ai is definitely no one hit wonder. she's one of the top5 female artists of Avex. (with Ayu, Boa , Kuu and Namie)