Personally, I'm a pessimist: although I'm pretty sure that they'll manage to get a single test flight in 2017-2018, I expect the program to follow the Ares trajectory and be cancelled sometime before its second test flight. By 2021-2022, the facts on the ground -- whatever they might be -- will simply be too clear. In a pessimistic scenario for the development of space, there will simply be no missions which the SLS could feasibly fly; conversely, in an optimistic scenario for the development of space, there could be many such missions -- but that same optimistic scenario necessarily includes cheaper commercial launchers, in-space assembly capabilities, and fuel depots -- all of which would provide for cheaper and more robust missions architectures than the SLS could provide. I can't imagine a middle-ground scenario whereby there is genuine demand for SLS launch services, yet no commercial capabilities to fulfil that demand. But hey -- maybe I'm wrong! Anybody want to tell me why?