I think Addai, when healthy, can be a very good running back. However, I don't think he has the durability to be a full time guy, which I was wrong about when he was drafted. If you look at his rookie year, and the first half of his sophomore year, the kid was very very good. That was before injuries took a toll on him, the OL, and even Manning last year.
This year, outside of the JAX game where the Colts running attack was pretty well shut down, Addai has been good, in some games great. The last two games he's looked far better than Brown IMO. Neither one has been given a full load of carries, so it's hard to say what would happen if Addai were (probably injured) or Brown were (who knows, maybe better?)
I think Brown was frankly, a bit of a reach by the Colts. I think they knew Addai wasn't a 300 carry RB, and wanted to grab a guy who was similar so he could fit the offense. I don't think Brown is super talented, and that had nothing to do with him going to the Colts and competing with Addai. I felt that way before the draft, and still do.
The fact that Brown hasn't been able to outperform Addai says more than I could say. The Colts coaches are going to put the best player on the field. They've never been a team to show loyalty to veterans, they often push them out the door when they're no longer useful. If they thought Brown was better than Addai, they'd play Brown more than Addai.
Addai, when healthy, is a top-10 FF RB. Right now in my one league, he's #13 overall RB, and in the other #16 overall RB. Clearly an RB2, while having about 10 touches a game taken by Brown. I'm not sure why I bother arguing with the people who insist he sucks.