Hate to nit-pick, but the Jets pass defense is hardly above average. It was a great performance, without a doubt. Absolutely not denying that. But he's had exactly one other "good" game, and that was a game in which I thought he was sub-par, and was made to look better by his WR's against a defense that I've watched play every week that really can't stop anybody.
Whether or not he's been good for the Patriots, I guess, is dependant on what the Patriots expect from him, but with that schedule the Patriots have played, their record doesn't impress me much, and while Cassell's played well enough to NOT cost his team games against bad teams, those performances wouldn't have been enough to beat good teams, and he's not a good QB, and he won't be until he can throw the ball downfield and not just boost his completion % by passing underneath all game.