I loved the first time I went to this museum. It's big, it's over the top and it's got some great unknown trivia facts about Texas. They also always have a changing exhibit. Make sure to check out their IMAX too. I've gone to see a couple of movies there and it's excellent. Their cafeteria is not too bad or overpriced either.
This museum is excellent for out of town visitors, specially if they're out of state. It exemplifies everything that Texas is about. It's a bit on the pricey side if you've got a mid to large-size group but you'll have such a good time there that it's totally worth it.
Tip for the tight-budget: Austin has a yearly Museum Day (usually in September) and most museums in Austin are free that day. Take advantage of this great event by going to museums that have an entrance fee.