As a regular Dusseldorf pax, I hear good things about Air Berlin from my colleagues over there. However my last trip with BA (about a week's notice - travelling out Sunday lunchtime, back last flight Monday) cost only £95.80 (inclusive of taxes), significantly cheaper than Air Berlin. Admittedly BA's idea of Sunday lunch was a Jordan's muesli bar, which sounds like that's one up to Air Berlin.
I'd like to give them a try, if only Stansted wasn't such a pain to get to from my home.