Anyone who argues we have a "good" pass defense is delusional.
However,
1) They've played against good QB's this year and held their own for long stretches of the game (Manning in the 1st half, Roethlisberger in the 2nd half, Romo for the entire game). Other than those three, they haven't faced anyone of note this year
2) If you want to know what a really bad defense is like, then watch Carolina play. They're 4-8 despite having the 8th ranked offense in the league.
The defense has been inconsistent throughout the year. They've shown they can play at a decent level at times, and at others they completely break down.
Only time will tell if they can get it together in time for the playoffs. I don't think Bill playing musical chairs with the secondary has helped.