Basically, I think defense at the PG position is a tossup. There just are not many great defenders at the position.
However, not many players have the physical gifts that Rondo has. He is not Steve Nash. He could be a dominant defender, and there is absolutely no excuse for him trailing after plays an getting caught on every pick like he does. Comparing to others is a paper argument IMO, because most of the others are being paid because of their offense...while Rondo is a nice offensive player, if he wants to be an elite PG (and lead this team to a championship), it is going to be because of his defense.