It is hard to come to the solid conclusion.
Some here like to make note of his "blown plays" in critical games ... but I stand by the notion that
Not everyone can make every play on every down.
If you look at W/L when he's playing 100% healthy you can argue the Bills have a much better chances of winning when he is in.
In the articles I posted he's mentioned as Quote might be underappreciated Unquote.