They went undefeated through the season with an average score of 37-13. They were far and away the best team in the league and generally dominated their opponents. However, The Giants managed to beat them in a huge upset at the Superbowl and the rest is history. I absolutely hated the Patriots, but there is no arguing they had the best team that year. But of course the best team doesn't always win.
That's one of the great (and awful) things about sports.