Holy.freaking.cow. If I could give this place a star every time I walked into a room and said "Holy sh-t," this place would have a 20+ star rating. Definitely the coolest thing I saw in Pittsburgh. I think I spent 2-3 hours here.
The rooms are seriously AMAZING. The pictures on Yelp don't do them justice. SO MUCH DETAIL is in each room from the floor to the ceiling to the door (even the outside of the door around the doorknob!!). Some rooms have items that are thousands of years old and should seriously be in a museum.
It's $4 for a self guided tour. You get a map of the rooms, an audio thing that plays info about each room, and a key to get into the rooms (the first floor is locked. The third floor isn't, if you want to check things out for free. Those rooms even have built in audio, so you'll still hear all the info. But really, fork over the $4 for the first floor. It's worth it.).
No air conditioning in most of the cathedral, so go on a day with good weather... And some rooms have annoying locks that won't cooperate. Most doors are pretty easy to open/lock (yes, you're expected to lock up when you leave a room) though.