Well, my own reviews have 4-6 comments total stating they had steered the commentor towards buying or not buying those books. Whether those people actually went with what they said or not I can't honestly say, though.
As for myself personally, before deciding on whether or not I wanted the Midnight book, I perused all the reviews here on ENWorld before making my decision.
Sadly, though, many of the books I've wanted a review on hadn't been reviewed when I was deciding on my purchase. Had there been, I probably would have paid heed to the review, depending on who it was coming from and how many there were.
As it is, I keep an eye out on the reviews and do base my purchases off them in part. Even if I don't agree with the rating someone gave a book, generally the review will mention enough of what's in it to give me a sense of whether or not it might interest me. I don't believe in blind purchases, regardless of the company, not now, anyway, so like to either browse through the book or read a review on it. Also, while I'm the only one in my gaming group who really reads reviews, as far as I know, being the DM has strongly influenced the purchases for my group. I've warned them off certain books and recommended others, with generally some effect. Sure, that's only about six people, myself included, but there you have it.
If reviews have a minimal impact, I'll say, for me at least, it has more to do with them coming out too late after the book's been published and my interest having since gotten the better of me while I waited to see one as opposed to the review itself being meaningless. The more timely a review, the more effective, I say.