This is an excellent, wide-ranging survey of the fantasy field. It covers everything you could think of: authors, films, television, comics, art, and major themes of the genre. As well as being highly informative, this is an encyclopedia that's genuinely entertaining to read. The editors have strong opinions about what makes good fantasy (ie not recycled pseudomedieval garbage), and that comes through strongly in some of the 'theme' entries. John Grant's movie reviews are also worth looking out for; they can be very funny at times!
Of course, it's not perfect. Any encyclopedia is out of date even before it's published (this only goes up to 1996), and something you look for is bound to be missing. Some entries attempting to define types of fantasy, or elements of its structure, can also be confusing. Many of these are written by John Clute, and he tends to use quite obscure words which will have you reaching for your dictionary in puzzlement. The entries on the structure of fantasy often refer to each other, making it hard to get a handle on the subject, espescially if you aren't familiar with the examples referred to.
Having said that, this is still the best single-volume reference work on fantasy, and I'm sure it will continue to be so for years to come. Worth reading, but a bit pricey..., so you may be better off with the paperback or looking for it in the library.