I think Rudi is a pretty big bust candidate - much more than DDavis.
As mentioned in other threads, he had 4 great games last yr - all against bad run defenses, so his total stats are really not a good indicator at all. CIN SOS against the run is simply the toughest in the NFL this yr - they will put the ball up early and often. Defenses will force CIN to pass more with the Palmer effect - make the inexperienced QB beat them. Finally - trading up to draft Perry isnt a strong indicator for Rudi.
As for your statement that DDavis is overrated because of a couple good games - I argue he had more good games than Rudi did. They may not have been as good - but he was consistantly more successful.
The biggest concern with DDavis is his frailty and a healthy Hollings pushing him.
Barlow wont get the numbers most people think he will - that is unless he is simply that talented where he can take on 8 defenders every play, and SF's D can stop opponents from scoring.
This is a big reason why the 2nd rd RBs dont have a lot of value to them - just a whole lot of risk.