PPG is good, but not a total picture because it's based on just points. I like to look at Defensive efficiency at football outsiders based on DVOA*
We are 4th on that list for total D.
3rd pass D
23rd rush D (eew, although I believe we are playing MUCH MUCH better run D now then earlier in the year which skewed the ranking)
FOOTBALL OUTSIDERS: Innovative Statistics, Intelligent Analysis | 2010 DEFENSIVE EFFICIENCY RATINGS
*DVOA is a method of evaluating teams, units, or players. It takes every single play during the NFL season and compares each one to a league-average baseline based on situation. DVOA measures not just yardage, but yardage towards a first down: five yards on third-and-4 are worth more than five yards on first-and-10 and much more than five yards on third-and-12. Red zone plays are worth more than other plays. Performance is also adjusted for the quality of the opponent. DVOA is a percentage, so a team with a DVOA of 10.0% is 10 percent better than the average team, and a
quarterback with a DVOA of -20.0% is 20 percent worse than the average quarterback. Because DVOA measures scoring, defenses are better when they are negative. For more detail, read below.