The catacombs feel like you are walking down a long hallway with no windows. They have lights. You walk some distance before you get to the bones. They do not allow flash where the bones are located. There are LOTS of bones. Its just interesting to see all the bones and how they made designs in the walls of bones.
If you know the story (mentioned on the website) or not the tunnels were the quarries where they pulled the limestone from to build some of paris. Then when the cemeteries started to fill up they started putting the bones underground in the quarry.

Anyways, I thought it was better than I expected. One thing, go early (get there 45 minutes before they open) or you could be standing in line for a couple of hours.