Orion drives are great if you just consider performance, but not cost. It gets it's great ISP and thrust at the cost of being very inefficient in it's use of fissiles. If you can lift propellant into orbit at the cost SpaceX is projecting, for any in-system work it is more economical to use much less flashy nuclear thermal rockets with much worse ISP, because the propellant is cheaper than the fissiles.
Other downsides include "using it for take-offs will leave a large crater that will glow blue for several hundred million years, as will everything downwind in the fallout area", but who really cares about takeoff areas? (That's for the silly plebs left behind on the ground to worry about - you're headed to SPACE!)
0: http://www.projectrho.com/public_html/rocket/enginelist2.php...
Once you have orions, you can move a lot of heavy stuff quickly around the solar system. Orions can probably go interstellar if we fire off fuel pellets for them to catch up to, or can somehow doe antimatter catalyzed fission/fusion.