I can think of one book I bought based on reviews -- Magical Medieval Society, and that largely based on Monte's review. There have been a few other PDFs for which reviews have added enough weight to push me from "eh, maybe" to "oh, I'll buy it."
I'm a pretty free spender when it comes to games, though.
I've also used reviews to decide to give one or two print products (e.g., a Monkey God adventure); but that was only after other buzz online had led me to seek out reviews of those products.
There have also products that reviews have warned me away from. It's possible that reviews have led me to decide not to buy more often than they've led me to buy. (Hey, I guess Hollywood is right to not let the press pre-screen really bad movies!)
Even then, though, reviews are usually a small part of the buying decision, and usually just a part of a more general buzz (e.g., a company, product, or author gets mentioned favorably in several threads).
Also, some reviews count for more -- I probably give more weight to Ken Hite's Out of the Box, a Pyramid Pick, or a review by Monte Cook than to most ENWorld reviews.