I've visited Space Center Houston several times over the years. I was in Houston recently and decided to visit again - unfortunately, this time I was disappointed with the experience.
Admission to Space Center Houston is not cheap and can be expensive for a family, especially if you also pay for some of the optional extras.
The Nasa artifacts (spacesuits, spacecraft, training equipment) are excellent. If you are a fan of Mercury, Gemini, Apollo, and Skylab, you can spend hours looking at everything on display. The included movies are also very good.
The Shuttle 747 exhibit is good, although not a huge amount to see inside the Shuttle mock-up and 747 aircraft.
The current tram tour around the Nasa complex is disappointing, compared to what you got to see in the past. A long wait for a tram and then a pretty mediocre tour of the main astronaut training facility. So many visitors on each tour, that taking photos is difficult and the tour staff just go through the motions and herd you along. The ISS Mission Control stop is worse with too many people being cramped into a viewing area to see and learn very little.
The Rocket Park (including a real Saturn 5) is the last stop on the tour. But you can visit Rocket Park for free. Just drive into the entrance to the Johnson Space Center and park in the visitor lot before the main security check-point. You can then walk into Rocket Park, see the exhibits and get back into your car, without waiting (hoping) for an open seat on one of the trams going back to the Space Center.
I visited on a Wednesday in March. The wait for the tram tour was about 50 minutes that day. When I got back to have lunch in the Space Center food court at 1pm, I could not believe the food line queues. It was not worth the wait. I finished looking around the center and then drove to a nearby restaurant for better meal at a lower cost.
If you haven't been to Space Center Houston before, I would say go if you have an interest in Nasa and space exploration. I would avoid the tram tour.
If you are revisiting the Center again, I would recommend not returning. It's expensive and has lost the excitment and individuallity that it had in the past.