Grading how NBA players do on defense is a very subtle art.
As a team, the Bulls' defense has been a lot better than people (including me) thought it would be. As KC Johnson tweeted, the team is holding opponents to the lowest FG% in the league and is 5th in Defensive Rating.
To find out how the guys are doing on a player-by-player basis though, we need to stop trying to rely on advanced stats and start focusing more on eye test.
Individual defensive advanced stats lag way behind offensive ones - defensive sieves like Carlos Boozer (5th in Defensive Win Shares in 2012) and Pau Gasol (8th in Defensive Rating this year) show up on the leaderboards all the time.