I have had this on my to do list for next trip to Paris for a while. I didn't want to go alone, so on my most recent trip there with 2 friends we organised pre paid tickets (with audioguide). The Catacombs are a must see, fascinating enough with the tunnel system, then the actual ossuaries. The audio guide is a must as otherwise there is not a lot of signs explaining various feature. It is quite steep down, and a bit slippery in parts so make sure you are prepared appropriately. It will take about 45 minutes to a hour.