We visited the Carrières de Lumières in mid-July and it was wonderful. My husband and I attended with our 12-year-old daughter and my husband's parents. Our daughter, who has travelled all over the world (no exaggeration), actually turned to me and said "this is my favourite thing on the trip so far. It's not like anything we've ever done or seen before." She loved the Gauguin/Van Gogh show but she *really* loved the Metamorphoses show shown in between. And my FIL, who has some health issues that lead him to be sensitive to noise/his surroundings and was going to skip the show and wait outside for us, was absolutely thrilled he changed his mind.
Cannot recommend highly enough and if you get a hot day, you'll also love how nice and cool it is in the quarries.
Get the combo ticket if you're visiting the Castle and if you park near the Castle or the near the quarries, you can walk between them - no need to repark.
Edited: 1:27 pm, August 09, 2012