I have been to Air & Space Museum at Dulles in Washington D.C. but Johnson Space Center was a different experience altogether, The Fascination I have with Space Travel, History of Space Travel, Apollo Manned Missions to the Moon, Mars Travel, All under one roof, it was simply Mind-Boggling, And A Dream come true.
We first went when Johnson Space Center opened up last year with appointments only, and we had a blast, I am an Aviator myself, and seeing all the equipment that was used in the Space Shuttle, Saturn V Rocket, made me excited yet made me appreciate the Humans that played their part in successful Missions to the MOON, and Equally playing their part right now to land Humans on MARS.
Fun fact, we didn't go once, but twice in 2 months, I'll go back again soon. It's a Wonderland. :)