The Football Outsiders just posted a ranking article on prospects that don't include first round picks and the well-known young stars. Via ESPN Insider:
Royster, a sixth-rounder from Penn State, put up the best rushing DVOA ever by a running back with at least 40 carries. Similar small-sample size studs of the past didn't necessarily develop into star backs. However, they did generally develop into useful part-time players, and Mike Shanahan certainly does have a history of getting the best out of his running backs. Royster also brings blocking skills and good hands as a receiver to the table.
It's hard to argue with ESPN's ranking here. Mike Shanahan, and more importantly, his long time RB Coach, Bobby Turner, have always found high value guys like this. Even though Hightower and Roy Helu appear to be ahead of the Rolls Royster on the depth chart, I expect him to see a lot more action. It'll be interesting to see in training camp if Hightower still has his burst.