Not intending to go to Pistoia, I was delightfully surprised when visiting this town and discovering a place that is not on the beaten path. We originally were drawn to the beautiful ceramic glaze frieze with figures depicting the acts of mercy on the hospital (worth the time to see) when we happened upon the Pistoia Sotterranea. We arrived at the start of an English tour. Our guide, Silvia, made the tour interesting using her sense of humor to highlight points along the way. Part of the tour that gets second billing was the medical school anatomy lab where dissections of human cadavers by medical students took place in the 18th Century. The room is a work of art, and we all wanted to sit in the room a little while. (I wish there was more information on the life of med students in the 1700s, how much money was required to attend a school like this, was this considered prestigious?) The subterranean tour is the main billing and piques your interest at how to dispose of garbage, did they ever clean the river (in the way we might now), was there ever a low river or shortage of water flow backing up dead animals or other blockages under the hospital, how does usage of the river for waste affect clean water supply, and how were they able to create such durable vaults 500 years ago.
I agree with another comment that "those looking for the wow factor will be disappointed, but we like to see things from alternate viewpoints so enjoyed the experience." This is one of those places. If you catch it on the right day and you are in a frame of mind to be open to it, it is a 5 star event.
If you are claustrophobic, you should consider if you will be comfortable. There are stairs and a low ceiling, someone with walking difficulties should ask questions before descending (it is not long, difficult or far but it is uneven).