I've been there twice, and will go back again. It's been a while since I was last there, 1994, when it just so happened that they had a live viewing of the Shoemaker-Levy 9 comet as it slammed into Jupiter. That was kewl.
They have a Space Shot liftoff simulator that will shoot you straight up, really high, and then you'll be weightless for a couple of seconds as you fall back down. Don't eat before you go on it.
They also have a hands-on wind tunnel (a small one) so you can learn the effects of trailer skirts.
The most impressive thing for me is seeing the full-scale prototype Saturn V rocket on display, inside of a building. It's laying down, but still.
I haven't taken it, but I know someone who has taken the bus tour. They said it's well worth it. They just restarted the bus tour a few months ago after shutting it down post- 9/11. It takes you to the Redstone Arsenal (which is an active military base, so the same security you're used to applies), and the Payload Operations Center.
Wear comfortable shoes, as unless you take the bus tour or watch a movie (I understand the Hubble Space Telescope movie is really good) then you'll be on your feet all day. Plan on all day, too. They open at 9 and close at 5. Get there at 9 or shortly thereafter.
The food is expensive. But the restaurant has a great view of the Rocket and Shuttle parks.
Incidentally,
free admission for two on Saturday, September 29 (to the US Space & Rocket Center, or any other Smithsonian Affiliate Museum in the country. Click the link to print out the free ticket (printer required).
