I've certainly had emails, letters and phone calls from people that I don't know, thanking me for drawing their attention to artists, albums and books that I have reviewed, but it would be interesting to hear from artists with albums/books released as to whether they think that positive/negative reviews make a difference to their sales.
Most of the people that I book at our folk club tell me that the large majority of their sales are not through internet, shops or distributors but through selling their merchandise themselves after gigs. It has been known for artists to take more money on their sales at our folk club than they have received in their agreed fee.