Impossibly expensive.
I'm a "rocket scientist" but I told my boss that if we really want to become a spacefaring civilization we should simply STOP developing new rockets (keep the ones we already have and use them) and put all of our effort into space elevators (skyhooks).
About the only "possible" technology that could make access to space cheap enough to allow us to consider really ambitious projects.
See Clarke's Fountains of Paradise or Hogan's The Web Between the Worlds.
Personally I think Hogan has a more believable scenario.
--Boris