I've done one myself and enjoyed it (for Dorion Solot and Marshal Miller's fabulous I Love Female Orgasm. But I don't tend to buy a lot of books, or rely as much on the reviews when I do.
When I'm looking to invest in a new vibe, I devour every review out there. I want to know all of the specs, how much power it has, is it waterproof, etc. But when I want to get a new book, I usually just grab whatever looks interesting (right now I've got my eye on The Feminist Porn Book).
So what do you guys think, is it even worth it to review a book? Do you bother to read the reviews when making your selections?