Statistically position wise though he is about equal to that of his offensive line, and has one of the worst passer ratings in the NFL at 24th (80.1). So I would agree that he has a bad offensive line, and knows how to overcome taking a beating and running for his life, but I don't know that it makes him a great QB.
He also had a much better line in the past prior to losing Faneca. I guess it again comes down to how much emphasis you put on a QB for a team winning games. I don't see where he really overcame the adverse conditions of his offensive line.
Might be a matter of how you look at it, but he's big, strong, and quite often has shown himself to be a playmaker. Remember the 3rd&9 and 3rd&12 in last Sunday's game? Yes he was 24th this past year, which I'm saying wasn't horrid considering that at one point he was being touched once ever 3 drop backs, and sacked once every 6. Last year he was 2nd behind Brady, and though he had Faneca, the line was still weak. He is one of the most sacked quarterbacks. Granted he holds on to the ball more than most, but still, the fact remains.
Since he has become the starter the Steelers have a record of 54-23. His rookie year he went 13-0, 14-1 counting post-season. Again, there are lots of reasons for this, Bettis, Ward, Holmes, etc. The fact remains, he deserves credit of his accomplishments, especially last year being a #2 rated QB in the league with a mediocre line, I think it was ranked somewhere around mid-20s. Further with a resume of 4 playoffs in 5 seasons, now 2 superbowls, and never having had one losing season, its still a consideration. Another note is that 30% of his wins have had a winning drive that he led. All I'm saying, is he is well above average.
I do agree though, and stand by my original point, that the Line is what can clinch or destroy a team. Look at last year's Patriots. Not many got to Brady, and he flourished because of it. Giants make the plays that get to him, and he throws a horrid game.