I used to think until I got season tickets and got to see some really great QBs play (especially compared to the dudes we trotted out). Brady makes incredible adjustments and he may not always have had the best arm, but he may understand the game as well as Manning and that think is far more important.
Best QB I've ever seen live was Brees, but Brady is a close second
Cassel started the year after they went 18-1. That was a really great team. No matter who they put in he was going to do well.