The catacombs are SO COOL! Well, hopefully you think the bones of a few thousand French people are cool (yeah, I know, I'm kind of weird). But it is a cool experience. Here is an explanation of my experience.
When you arrive outside the little building, you have no idea how big the place actually is. You get inside, and walk down what seem to be an endless array of spiral stairs (By then, I was quite sick of them, I had just been to Notre Dame and St. Chappelle), you walk into a labyrinth of hallways filled with the bones of thousands upon thousands of bones set up in an almost grisly display of what the plague forced people to do with their dearly departed.