RPG.net added a hit counter to their reviews a couple months ago. The results were a lot less than I expected.
Generally speaking, most the reviews I've written have ranged from 900-2000 hits, most generally around 1500. (Actually, I wrote down how many reads my had the other day...):
As of Midnight, August 21st (the date the review was posted in in parentheses)
Into the Blue (June 9th) - 928
OGL Horror (June 11th) - 2106
HPL's Dreamlands (June 25h) - 1300
Legends of Excalibur (July 9th) - 1685
OGL Cybernet (July 19th)- 941
Dungeon Crawl Classics 7 - Smuggler's Cove (Aug 9th) - 1421
d20 Future (August 20th) - 886 (That was in 1 day, it's now up to about 2300)
They also have a top ten list. The top is Eberron, at 8500, then Lone Wolf at 3500, then a bunch at 2500 or so.
Now, even if 10% of the readers happen to buy the product in question (which is a very very generous guess, I would really guess the true figure is probably closer to .1% and even that is a wild, wild guess), that's not very much in most cases. At least from a larger companies' perspective.
OTOH, for really small companies, and PDF companies, they do help.