There is already a good review and some brilliant comments about this book. However, having just read it, I'd like to chip in by saying that I think it's both a fantastic read and an important contribution to human understanding, no less.
As mentioned above, it picks up where Terence Mckenna left off. Two of the ideas introduced by T.M. were: 1/that hallucinogens (specifically psilocybin-containing fungi) may have played a vital role in the evolution of human culture and religion 2/ that entities sometimes contacted with the help of psychedelic substances may not just be 'projections of our unconscious', but may have their own independent existence.
Mckenna's writings are fascinating but frequently highly speculative, so it is easy for the sceptic to dismiss them as ramblings from yet another wacky drug fiend. In 'Supernatural', however, Hancock has gone to great lengths to produce a large body of evidence to support his ideas. This is why, I suspect, he gives so many examples of Upper Paleolithic cave art and rock paintings from southern Africa. His work therefore screrams out to be taken seriously, and is not so easily ridiculed by those who might find some of his truths rather inconvenient.
Of the several major threads in the book, I would say that Hancock is convincing about the following: 1/ Ancient cave art, more modern rock art from southern Africa, fairy stories, alien abductions, and experiences of entities on DMT, ayahuasca etc are all tapping into the same region. 2/ these are all intimately connected with altered states of consciousness, most reliably (though not exclusively) accessed with the aid of psychedelics. 3/ The use of psychedelic substances, and shamanic devices generally, has played a much larger part in the development of human culture and religion than many people will fel comfortable with.
As for whether the fairies, therianthropes (those half-human, half-animal beings), and other entities are 'mere hallucinations', whether they have been hard-wired into our DNA from the beginning (radical idea), or whether they 'really are out there', the jury is still out, despite the topic being investigated thoroughly, including a substantial interview with Rick Strassman.
So, yes, for those interested in religion, visionary art, shamanism, psychedelics, aliens, what-are-we-doing-here-anyway?, this is a must-read.