I was pleasantly surprised by how fascinating this turned out to be. I visited because I like the history of alchemy and thought the museum might be interesting. I was a bit wary. But I was met with a look at an original alchemist's laboratory, not a New Age or even touristy tribute to a historical pseudo-science.
The visit was comprised of original lab facilities (really, very cool) and a reconstructed library. This was one of the few things I saw in Prague that was more geared toward history and knowledge than over-wrought tourism. The personally-guided tour was fantastic as well, with a well-versed guide who answered questions and offered a complete and interesting overview of the museum.
In short, a unique, magical and unusual place to visit.