We've had the discussion many a time of how he's the best QB they've ever had, but that's not saying much, nor does it mean he's anything more than an average QB.
But like I said it's to be expected. When you've had such poor play at the position for an extended period of time, when someone better than all the rest comes along, fans generally overrate them. It happens in different sports as well.
Take the NBA for example. When the Lakers first got Ramon Sessions, Laker fans overrated him like crazy and to them he seemed better than actually he was, just for the simple fact he was an upgrade over the Derek Fisher's & Smush Parker's they had the 10+ years before.
As for Flacco himself, I'd say he's a borderline top 10 QB. I think he's a solid player.