Brady doesn't play a heavy running / movement position however. His knees could be so-so and he could still be fine, standing in the pocket like he always does, making his checkdowns and then breaking records.
I work in Massachusetts, so the sports radio up there is fixated on this 'what can we do with Cassell next year' thing. He was thrust into a system which yielded record breaking numbers last year, so of course he can produce numbers and big games. The weapons around him are what makes him look so incredible.
One thing I do know - there are some wacko Patriots fans out there who think they should trade Brady for a bunch of first-round picks and stick with Cassell. They don't get much talking time before their blown up though..