Mid-level Pratchett, not up there with INTERESTING TIMES or down there with ERIC. The somewhat claustrophobic action takes place entirely in the Ankh-Morpork Opera House, and Pratchett is mainly out to satirize opera, opera singers, and opera lovers, as well as all the variants of Gaston Leroux's PHANTOM OF THE OPERA. The hard-to-take Granny Weatherwax dominates the action, what little there is of it. The ostensible main character, "Perdita X Dream" as she calls herself, never comes to life for a second, or has much of anything to do in the novel's development.
There are walkons from several Discworld regulars, such as Nobby and the Librarian, but by and large this really isn't a Discworld novel--- that is, it could take place anywhere. It is difficult to figure out how Pratchett wants the reader to take some of the humor, and some of the apparently serious moments. For example the villain has a long, operatic death scene in which he berates opera virulently, in a perfectly straight tone. Is he speaking for Pratchett? Apparently so, since the omniscient authorial remarks about opera are in pretty much the same style.
Anyway, Pratchett is clearly having some fun with opera and it is unfortunate that the reader is not likely to have quite the same level of fun, to say the least.