I think he'll be a top 15 back. That being said... You won't be able to "steal" him in any drafts, because he DID play well last year when given the starting job, and the hype train on him will drive him into the late 3rd early 4th round... which is probably fair value for him anyway. Right now, in march, his ADP is early 6th round... but if inevitably DeAngelo is sent packing, expect Stewart's ADP to skyrocket.
As far as his injury issues, I don't see him as great an injury risk as Gore, MJD, DMC, Bradshaw, Best, or Moreno... and all these backs will likely be taken ahead of him. I truly think he has as much upside as any of the above backs, I like him more than Moreno, Bradshaw and Best...
If you go RB1, WR1, BPA, in the first 3 rounds, and can get him in the 4th as your RB 2 this fall, I think you've got yourself a solid foundation.