Google is successful because it has managed to hold onto being the premiere search engine and then selling the keywords that it sees are the most popular. "Word of mouth" has literally almost *nothing* to do with it.
By "reading reviews" you mean in regard to ebooks, certainly, but SOMEONE is going to be first and is going to have to fork over the stupid $.99 to find out if what the author claims is super amazing awesome ZOMG!.. is really just crap. It gets more complicated because the author can log in with a different account and post 5 star reviews and praises to muddy the water. If a spammer only gets one or two sales before someone posts a negative review, then they still win because they are doing this on a massive scale. They've got like a thousand crap books out there, all getting 2-3 sales. And those books don't go away, they just sit there in the listings wasting people's time and bandwidth.