If you are a fan of C.J. Carella's WitchCraft then this is a must have book. If you are a fan of his work in GURPS Voodoo, then this is a must have book.
AC extends WitchCraft to include True Immortals, Ferals (the Weres) and the various servants of the Mad Gods. We also get a little voodoo in the form of the Covenant of Legba.
Crunch wise we get some new skills, qualities and some weapons. There are also a new branch of metaphysics, the Lesser Keys of Solomon and Voodoo.
Not required for WitchCraft (it is still a great game all on it's own) this book does enhance it quite a bit.
I of course think it is a must-have for any game.