I've read lots of reviews on here and Facebook from people saying they could go back multiple times to the NHM. It was a great experience, but definitely a one-and-done for us. It was extremely warm and busy when we visited, which didn't help.
Don't get me wrong - there were some really breathtaking moments. The first is when you enter Hintze Hall and see the blue whale skeleton suspended above you. The second is entering the mammals hall and seeing the models of animals, again including a blue whale. The third was the incredible escalator through the centre of the planet. Dippy was also brilliant and that exhibition is well worth seeing!
We also loved the animatronic t-rex in the dinosaur section (extremely busy) and the earthquake room! These were really cool interactive experiences and the museum could do with more of these!
The cafe was expensive but had good choice of hot and cold food, and it was nice. If you can, take your own food and have a picnic out on the front lawn - plenty of people were doing this when we visited!
The exhibits were interesting, the setting was impressive and I'm really glad we did it, but I don't feel any great urge to return and see the bits we missed out on. Still, it was free and we had a great time!