One of the very best space museums on the planet. If you're a real space nerd, you will go home happy -- the collection here is incredible. That said, it's not very well organized. The main problem is that many of the best things to see are on the "tram tours." There are three tram tours:
1. The Rocket Park: You can see several rockets here but by far the star of the show is the Saturn V, the biggest rocket ever built, and one of only three left in the world.
2. The Apollo Mission Control Center: You get to visit the viewing area of the real room where they guided Apollo 11 to the lunar landing. They've restored it to just how it looked in 1969 and they have a presentation where all of the consoles and radio communications look and sound exactly like they did during the first lunar landing.
3. The Astronaut Training Center: I wasn't able to do this one so I can't say much.
The main issue you'll run into is that the trams that take you to these places are very poorly organized, loading is slow and inefficient, and many trams sit around doing nothing for a long time. This means that you need a reservation and you will spend a lot of time waiting in line for a tram.
You can get a reservation either at the visitor desk to the right of the entrance, or through their app. You should download the app before you get there. You can only make a reservation with the app once you're at the space center. You can only get one reservation at a time (i.e. you can't get a new reservation until you've gone on the tour) for each method, but you can get a reservation on the app and from the visitor desk simultaneously.
The first tours to book up are the mission control ones, then the astronaut training ones, then the rocket park, so if you want to visit mission control, I would book that first.
If you come late in the morning, you'll probably get an afternoon reservation for mission control, so you may be able to also book another tour before mission control, but leave plenty of time. It takes about two hours from your reservation time to wait for the tram, see the rockets, wait for the tram to come back, and get back in line for your next tour. If you want to see everything I would try to arrive at opening.
Most of the really cool artifacts are in the Starship Gallery. They have the Apollo 17 command module, moon rocks (including one you can touch), a Mercury (Faith 7) and Gemini (Gemini V) capsule, a Lunar Module, and you can walk through a very realistic training mockup of Skylab.
On Independence Plaza you can go in a very realistic mockup of a Space Shuttle and also the Shuttle Carrier Aircraft.
Overall a lot of fantastic things to see, but get there early, plan ahead, and plan to spend the day if you want to see everything.