I'm thrilled that Rex is back in the division. It will give me more of a reason to hate the Bills, at team I'm currently finding myself rooting for more than I should. I'm glad his back in the division because he's a bad HC, who seems not to be interested in improving his weaknesses, but rather trying to prove that everyone else is wrong about him.
On the plus side, he'll be a PR asset, and entertaining as all hell for the fans and media. The defense won't get worse, and his players will like him better than Marrone. On the negative side, his game management will still be horrible, his attention to detail will still be lacking, his commitment to accountability still won't be there, and finally in the end, his dedication to his personal self promotion will always take precedence over building a team that can win long term.
I think he's taking the Bills job because that's the only one he's been offered. I think Blank/Dimitroff/Pioli are too astute to offer their HCing job to Rex. They see through the personal charisma and are aware of his fatal flaws as a HC. If Rex wanted to go do what he's best at, he would have taken one of those TV jobs