The Importance of Being Ashlee has always been at the core of the younger Simpson sister's musical output. (See her first two releases, insistently titled Autobiography and I Am Me.) Yet four years of stylistic switch-ups and alleged cosmetic surgeries later, her identity Punk-Pop Princess? Dance Diva? Husky Balladress? remains an enigma wrapped in a riddle and swathed in hair extensions. From the first notes of Bittersweet World's opener, ''Outta My Head (Ay Ya Ya),'' with its mock-ska inflections and '80s-redux backbeat, it's clear that her latest incarnation is Gwen Stefani, The Solo Years.
Though it's tempting to dismiss Simpson as an empty shell, she can't seem to stop turning out perfect pop nuggets. Bittersweet brims with
polished and similarly Gwen-esque hooks (the loping, exotic ''Murder,'' the synth-driven ''Boys'') and pretty melodies (melancholy ballad ''No Time for Tears,'' well-wrought breakup anthem ''Little Miss Obsessive''). Producers Chad Hugo (of the Neptunes), Timbaland, and Kenna have imbued the album with a giddy neon energy, and Simpson responds in kind, gliding through glossy grrrl-power-lite sentiments and saucy club come-ons. It's only on howlers like ''Rule Breaker'' (''Some say I'm a black sheep/Cuz I like to sleep late''? Girl, you so crazy!) that she stumbles too obviously. Still, even if Simpson will never be considered an authentic artist, she remains a surprisingly fillable vessel. B
DOWNLOAD THIS: ''Boys''