The Catacombs experience is really eerie seeing all the skulls and bones piled up in the underground mines. As you go through the different chambers and passages you read about the history of the mines and the making of catacombs.
It would seem that they limit the amount of people who are underground for safety reasons which is good and so that its not over crowded and so that you get a better experience of being down there almost alone at times which is great. The downside to that is massive queues to get in. So be early and come appropriately dressed for waiting a long time outside. There is a McDonalds down the road and a small coffee shop that sells coffee and snacks which is good.
It was an epic experience for us and we despite the very long wait to get it we really enjoyed it when we walked through. We would certainly recommend see this if you are in Paris.