We visited in the Easter holidays, but did not find this attraction overcrowded. It is really smoothly-run, with lots of helpful staff available, and plenty of information. The exhibits are dramatic and interesting, and presented with lots of space around them so you don't feel crowded. The creators have selected wisely, and the tour is not too long (around 3 hours) so even our slightly older child did not get bored. Surprisingly, the food available is good-quality and healthy (nice sandwiches) and very reasonably-priced (a good doorstep cheese sandwich for £2.75). The facilities are very clean, with really good toilets and parking. You are not bombarded with extra costs (apart from pricey DVDs/photos of your child flying a broomstick, but these are always pricey at theme parks etc.), or tacky things to buy. The gift shop is vast and has pretty good-quality stuff; it is expensive and hard to find anything very cheap, but not as bad as we had feared. I would really recommend this day out. It ends with a particularly breathtaking exhibit which blew all our minds!