He hurt a hamstring early in TC and never got a chance to really compete against JP for the starting job. However, he looked good in the games that he did get to play in. Both Nall and Losman are about the same size with strong arms and good athleticism. Losman's arm is better, and he's more mobile.
Is Nall starting QB material? I don't think anybody knows because he's never gotten a chance. After watching how the Bills QB coaches improved Losman's fundamentals last season, though, I'm sure that Craig is also much better at fundamentals than he was before he came to the Bills.
I don't think he's going anywhere this season at least.