This book had a great deal of influence on gaming in general (including the revelation that, if you give something hit points -- even a god-- PCs will try to kill it, and often succeed). The first printing had an even greater influence on the campaigns in which I was involved in High School, because of the inclusion of pantheons based on the works of H.P. Lovecraft and Michael Moorcock. Because of a dispute with http://www.chaosium.com/, who published licensed RPGs based on those settings, those chapters were excised from later printings of the book.
For those of you interested in a trip down Nostalgia Way, the entire contents Melnibonéan Mythos chapter is now available on the World Wide Web, complete with the original art (where the links are not broken, alas).
On Michael Moorcock's own web site.
Cool Factor = 11 on a 10-Scale.