We don't rate many things as excellent but this deserves it. But you do have to be smart about deciding what you see when, as not much effort is made by the Centre to make it easy to see everything.
Firstly, the Tram Tour is a potential headache. Avoid waiting in line a long time by going on it as the first thing you do if you arrive early (which you must if you hope to see all there is to see in one day). NASA may be able to get people to the moon, but they apparently cannot efficiently design a tram tour. They have only one access gate to board the tram train which has about 30 rows of seats. 30 access gates would do the job in one tenth of the time. You are invited to line up for either a red or blue tram tour. No explanation of what the difference is until you reach the doors to go outside when you suddenly learn what is covered in your tour. How about a sign where you join the queue so you can make an informed decision. Oh, after the brochure & website encourages you to book (& pay entry) a timed ticket for the Tram Tour, guess what? Absolutely NO attention paid to timed tickets - everyone just lines up for a ride when they feel like it.
That's the bad part. All the rest is good. Retired NASA employees give excellent explanations of some of the exhibits, but we couldn't see anywhere where the schedule for these appeared so you could plan to hear them. Other scheduled presentations in the various theatres are flagged to you in a brochure given when you buy tickets. On tickets, be aware that while the ticket office may open at 9am, the exhibits don't until 10am, so you have a wait.
You need to plan your day according to the scheduled times presentations you want to see are held. Some are repeated and some are not. No-one advises you where to go in the very large exhibit building and although you are given a map, it is not always obvious where to go. For example, it is not obvious that to get to the best exhibit area, you have to go through their largest theatre to access it (Destiny Theatre i think). Not obvious that to get to the shuttle-carrying jumbo airplane, you have to go through both of the formerly mentioned areas.
Bring your own lunch if you don't want to help fund most of the budget for their next launch via the very expensive Zero G cafeteria. Ditto for the gift shop.
Allocate a whole day (10-5) if you want to see everything.