I'm in two minds about the X Factor USA rumours.
On one hand, it would/should be great exposure for Britney and a chance for her to come out of her shell as a confident public celebrity. We would get to experience loads of Britney during a time where she would have no music in the pipeline for a long time. Britney's career has been built on the definition of having "the X Factor" so in that sense she would bring something vital, something which Paula or Nicole didn't have. Plus, despite hating it last year because there was no spark to the show, I have a feeling Simon and the producers will bring it with quality this year. Who knows, she could become the nation's sweetheart again from the show?
On the other hand...

I can see it going awfully. Britney could freeze up when it comes to giving criticism and a flood of "that was very cool, I liked it" would come from her mouth about a hundred times per show!

Somebody brought up the possibility of her being labeled a has been too, I can see that.

The X Factor USA isn't at an A-List International level yet (I think that's why they're throwing Britney's name into the media as often as possible) so it would be deemed a step down or as a "desperate attempt to save her career"! When you think about it, she's replacing Paula Abdul or Nicole Scherzinger...! Then there is the singing ability accusation she will undoubtedly face time and time again, be it from the media or disgruntled audition rejects, which she will unlikely be able to defend herself against. I cannot see Britney going "I dont sing, but I dance..!", she'll likely get upset and go further into her shell. With Christina on The Voice beating X Factor USA too, it would compromise her position as being seen as a more successful popstar than Xtina.
It's tricky...