We will only know if they are good or great by the D's performance in the playoffs. If we get there.
I agree, but their are indications that the Jets D is one of the top D's in the NFL this year. The season is a long slog and there are too many variables to judge a D on a game by game basis. Too many variables. Playing teams with nothing to lose. Lucky bounces. Etc. IMO, the best measurement is points per game and the most points given up in one game. The most the Jets D has given up in any game is 23 v. Miami. So, they really have not laid an egg this year. See Baltimore v. Buffalo for an exmple of an egg by a supposed top D. And their scoring avg is third in the NFL. So, despite the concerns, they have the numbers od a top D. When things "toghten up" in the playoffs we will know for sure.
Interesting note. The D's two best performances by far --- giving up 9 points in games against the Raven and Packers --- resulted in losses.