I watched an NFL.com episode where they ranked the top 20 RB's in the draft and they had a guy from Utah State and 2 guys from non-BCS level... and no Rex.
I wager that after say 3-4 years, were you to look at how well the RB's from this class contribute at the NFL level... that Rex would be in the top 20 and likely the top 10.
So... if he goes as low as some predict... I think he will be a steal.
Its mostly because of what he was not capable of doing his Senior year. Marcus Lattimore should easily be a top 3 RB, but due to his injury, there is a lot of skepticism on what he can do at the next level, and how long he will last. He did have a much better career than Rex, but the same questions go for Rex, even though his injury wasn't close to being on the same level as Lattimore's. Had Rex been able to have a solid year, injury free, he would be a 3 rounder with ease.
Down the road, I think Rex will be that quality back for whatever team he lands on. He could end up being traded a couple times, until he finds that offense and coach that he meshes well with, and they know what he can do. Like I said before, he isn't a Frank Gore, Eddie Lacy, Trent Richardson, Arian Foster or Adrian Peterson, so I don't expect to see him as a big time starter, but he will be a quality, reliable back.