I'll admit, when I first heard about this, I was skeptical. I thought, "a bunch of old bones organized to make 'art'?" Yeah, whatever, I'll pass.
However, as I thought about it, and how the same person that told me about said, "It was really cool though," my curiousity got the best of me. I'm REALLY glad it did.
Yes, it's still a little creepy walking through the rooms filled with bones and I wouldn't recommend it if you're freaked out by bones, but the designs are tremendous. It's well-lit, and not intentioned in any way to be scary or haunting. There are no moving parts, other than the wind, just designs made of bones from the monks of this brotherhood.
It is "free", but a donation is accepted. Even still, €1 is a cheap cost to see this.
And, a fun little (light-hearted) story. According to legend, an unknown coffee shop down the street started making a different type of sweetened coffee. The color of this new drink reminded him of the color of the cappuchin monk's hoods, therefore, it they named it cappucino, which literally means "hood" or "something that covers the head".
When visiting Rome, this is well worth a visit.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.