Britney - I'm a huge fan but I'm not sure if the X Factor is really for her. She seems to be a decent mentor and while her judging comments are short and to the point I don't think she's half as bad as everyone says. I just don't think she's suited to judge others, it's not in her personality.
Demi - I love her banter with Simon and she provides some much needed excitement to the panel. Her mentoring has been pretty bad if I'm honest though. She had amazing acts in Jennel, Paige and Cece and she pretty much ruined them. Your highest act finishing in sixth place isn't good at all.
I think Demi should be back next year as L.A. is leaving and changing the judges around too much gets confusing. Hopefully she'll learn from this year and correct her weaknesses. Britney doesn't need the show and with an album coming out next year she should focus on that.