Manning is not overrated. He's one of the best ever, and if someone says otherwise, I would seriously question their football IQ.
When I say he's overrated, I'm not saying he's garbage. I'm just saying that when a playoff game is on the line, I'd select Brady, Roethlisberger, Brees, Rivers, or a retired Kurt Warner before him.
To be considered one of the 'best ever', you need to at least be in the top-2 or 3 of your era and he has not been that guy in crunch time. The regular season only matters in college football, and as we all know, his Tennessee team never beat Florida.