In commenting on Adrian Peterson in one of the fanposts:
Looking at DVOA (though its early in the season so stats can get skewed) and Success rate you see that his Success Rate is not as high as his overall DVOA which tends to indicate that he gets more big runs but has a higher number o runs for ineffective yardage. Which as stated can lead to more frequent 3rd and long situations. Coupled wth a weak QB (Jackson) you get an inconsistent offense.
I realized that this brings up n interesting question. As a matter of football strategy (NOT fantasy football): Would you rather have a Boom/Bust RB (one who can break long plays on a semi-regular basis but who also generates lots of low to no yardage running plays) or a "Boring/Consistent" RB who consistently generates 3-6 yard runs but rarely has low or negative running plays but also rarely if ever breaks long running plays.
I think this argument is greatly affected by consistency and strength of your QB (i.e. Jackson kills the Vikings because Peterson's Boom/Bust tendencies frequently means that somewhere in a drive they are likely to face 3rd and long which more dramatically increases Jackson's influence on success).
I think with a Peyton Manning a Boom/Bust RB is the best option because it puts the most pressure on the defense and you still have Manning to bail you out of 3rd and long when needed.