Dear unsatisfied customer,
I know how you feel, having bought goods and services myself and found them wanting. The price doesn't enter into the equation and I recognise you've chosen to spend your money on my book. Thanks for that initial vote of confidence.
I read your review with interest - as you can imagine, I read them all! I can't speak for your experience because, let's face it, nothing has that fingernails-on-a-blackboard feel to it like an author trying to justify their work to a critic. And it could be argued that if I have to point out the good bits, or what I was trying to achieve, that's as good as admitting I've failed.
However, while I'd never knowingly attempt to change a person's mind once it's made up, may I offer some advice? If a book has really disappointed you, when you review it why not let the author - and other readers - know where it failed and why? That way your negative experience can be turned into a positive by giving the author something to work on with their next book.
I hope the next book you choose is more to your liking. Incidentally, I have other books out there that you might enjoy, or not!