Just so we're clear, I wasn't expecting the Paris Catacombs here. I'd done some research and knew…read morethat this was a small museum without a lot of exhibits, so my expectations were managed. Even so, we were kind of surprised by how little there was to see here.
Many reviews mention that the entrance is hard to find, but we didn't think it was that difficult -- however, it's important to note that you'll be looking for a gate in the middle of the sidewalk with stairs leading down (like a subway station), not a building. There are actually two entrances and we found the closed, chained one first -- the actual entrance will have a green museum sign, as pictured.
Once you go downstairs, you're pretty much right there. There is an open area off to one side with a series of vaulted stone arches, and you can walk through the empty room looking at the layers of stone and brick that made up the foundations. But you won't find any gravestones, plaques, or artifacts, let alone any grisly skeletal displays -- it's literally an empty room. It looks as though the central area is left open so that they can have musical performances, and one of the entrances seemed to have a program of upcoming choral and classical music events, which is probably a neat experience.
Strangely, it looks like the crypt used to have a lot more as far as signs, exhibits, art, and artifacts. If you look up the circa-2011 YouTube video tour by Susana Provenza, you'll see plenty of explanatory signs and displays around the crypt, but none of those were there when we visited. Honestly, watching this video might be as much of a visit as you need, because it doesn't look like that today.
Admission is inexpensive and their bathrooms are acceptable, so if you feel like spending a couple of bucks for a quick rest stop, the crypt won't take up much time. But don't feel bad if you don't get around to seeing this "museum."