Wow, so many useless answers here.
Many say that he will struggle because of a much weaker offensive line on NYJ than he was used to in PIT, and this is the only knock I think is valid.
Many also say that he will struggle just because he's in a new offense and team, but that seems baseless. Many also say that he will struggle just because he hasn't played in a while, with no injuries during that time and fresh legs...so I disagree. Many also say that he won't run much because of the coaching scheme, but that's based solely on last year without looking at what he did beforehand.
So in conclusion, I think most of the criticisms are invalid, and I think most people are forgetting how special his talent is. I think he will be the 5th best RB