Christina is obviously talented - my only problem with her is that she doesn't know who she really is.
When she came out with her first album, she was sorta following in the footsteps of Britney and all the other teenage pop acts out there. She had some hits, but I think it was hard to connect with her.
Then she came out with Stripped, being all dirty and slutty. Totally not her. And again, following in the footsteps of Britney's "Slave 4U" and the slutfest that was going on in pop music at the time.
After that was Back to Basics...which sounds a lot like Amy Winehouse's Back to Black, which came out the same year. Nearly identical title, similar 20s sort of stylings. Not to mention, Christina's album sounded nothing like her first two albums. Is this the real Christina this time? Who's to know?
Now she's got Bionic, which a lot of people are comparing to Gaga's The Fame and Fame Monster. All Christina has done is follow the trends. I still have no sense of who she is, what she's about. That's not to say she doesn't have an amazing voice - she does. On par with Celine Dion and Whitney Houston. But it seems like Christina's whole career has been a game of catch-up, and she'll never top the charts unless she does her own thing and stops trying to fit in.