This simulator, at least the last time I was there, was at the Udvar-Hazy Center and not the NASM on the National Mall. Perhaps it has been moved.Its a sit down ride, about the size of a large SUV with I believe three or fours rows of benches. Once inside, the door through which you entered lowers to encapsulate you in the darkened "theater". The entire cabin is sitting on a hydraulic lifts that provide pitch, roll (though not 360 degrees) and yaw.
Riders where 3-D glasses and sit forward looking at a screen (roughly the size of a large plasma screen).
The movie itself has good graphics and the movement syncs to what you see on the screen, but from a realistic viewpoint, it makes several mistakes and invents scenarios that could never occur. If memory serves correctly, the premise is that you are sitting in a crew pod mounted in the shuttle's payload bay (you separate once in orbit but return to reenter in the orbiter).
One example of an unrealistic element that could likely have been easy to include but wasn't involves the landing. As most know, the shuttle flares before touchdown at a rather dramatic angle. There is no attempt to recreate this in the simulation even though the same movement is felt earlier in the ride.
Everything taken into consideration, I'd recommend taking the ride if you have the opportunity, but I probably won't repeat the experience unless accompanying guests new to the UHC.