We built a 30x36 Morton pole barn on our land in 2003. We finished 2/3 of it into a weekend cabin, and have spent weekends there since.
Tom is correct, in that you can finish the interior however you like. We're planning to build a 2300 square foot house using a Morton shell. They build the exterior walls on your slab, and will install the doors and windows. They will also make the walls and ceilings ready for drywall and install insulation. With the house completely enclosed and dry, you put up your interior partitions, wiring, plumbing, drywall and finish. Since the shell will be very cost-effective, we're planning on spending more for a nice interior.
Since the walls will be over 9" thick, there's lots of room for insulation, and we expect it to be very energy efficient.