I think comparing him Faulk isn't giving him enough credit. You could take Faulk off those teams and they would still win as many games IMO. He has been our Welker (little engine that makes the offense work) frankly our Brady (best offensive player for years) though obviously Brady has far more impact.
We have some other talent at RB but Jackson has been the cornerstone of our offense for years. Take away Spillers 2012 (breakout?, fluke?) year and there wouldn't be any question who has been the bils best offfensive player this decade.
On the speed issue I do think he was faster in the past. Back when he was backing up lynch we would call him fast Freddie every time he got the ball. I don't see that now but I think he has gotten stronger over time.