Guy is talented but as others have said he needs a good supporting cast and that's fine, only Top QBs can pull off miracles of having a winning record with garbage teams.
I'm not sure how he would've done in NE, different environment, different development. We tend to think most players would develop better here with Belichick but SF is a good organization and Shanaham is Ok so I don't know, Pats football operations for the past few years has not been so good as it used to be.
Jimmy is made of glass and sometimes just have brain farts like that fumble against Green Bay 2 weeks ago, he did well later and put his team in position to win but they lost and that fumble cost points. In that division it could cost a playoff entry as well.
Overall I think he has a place as a starter in this league and he can be a capable one, but I have the feeling he is entering that stage QBs get into a downward spiral mentally and never come back, Andy Dalton was a top QB on that 2nd layer of QBs after the Bradys and Peytons once, now he is toast, Jimmy can go there really fast with another injury or two.