I love the tracks "Mother" and "Skyfall" on the Skyfall soundtrack. I love the rather esoteric mood in those. That's something I can't imagine coming from Arnold, he would have scored it totally differently.
And yeah, it's kinda telling that for one of the key moments, the revelation of the DB5, they used a track from David Arnold's score from Casino Royale.
Same thing happened in GoldenEye in the tank chase. Eric Serra's score sucked for that scene, so they used a track from one of the earlier John Barry scores.