Add Arian Foster and Peyton Hillis to that list too. They have proved probably less then Jackson has in their time in the league. Yeah, big years last year, but let's see how this season goes.
I don't think you can rate RB's like you rate QB's because the talent level is a lot more even. I think you've got the elite in Peterson, Johnson, Jones-Drew, and Turner (possibly Charles but he's out now), then from there the rest are all very good (with McFadden, McCoy, and Ray Rice set to take over).
Basically, if you have Jackson, you're pretty happy about it. He hasn't put up the otherworldly numbers that the best of the backs do, but he's going to give you 4.5 yards per carry and about 35-40 receptions.