To be honest, I never really thought about how much some of my purchases are driven by reviews, whether it be on Amazon or a specialized site, like one that handles computer hardware reviews. It wasn't until I started writing that I realized how much reviewing, well, everything could matter.
Books for example; I never considered reviews. I generally read the blurb on the back (or on the web, as it were), maybe flip through some pages for writing style and decide on the purchase right then and there. Even now, I only go through the lowest reviews and skim them for consistent complaints. If it's a serious complaint, I may shy away from the book. If it's something that I can tolerate, I'll probably go ahead and buy it. My thought process and research goes far more in depth when it's something more expensive, like computer parts, or a car.
That being said, book reviews need to happen a lot more than they do. I had no clue how much people relied on those to make their choices. As an indie author, I am pretty much a no name person in a sea of no name people. I rely on my own ability to advertise, which is lacking due to minimal funds, and on reviews. Reviews drive my sales. Basically, it's printed word of mouth. I've had several sales where I've been told that if it wasn't for the reviews they would have never picked up the first book...which followed with the 2nd and 3rd book. The biggest reason, for me, that reviews make all the difference is the blurb of my books. If you've read them you understand fully why I say very little in those blurbs. My books are non-stop, story driven craziness. Even small things that seem like nothing end up being something more later down the road. Because of that, it's very difficult to write a blurb that doesn't ruin some aspect of the books.
I am not saying that you should only write 4 and 5 star reviews. Bad reviews are just as good. It helps the author know what people dislike, as well as potential buyers. However, if you don't have a real critique then please don't review. I've seen several now where someone simply said, "I didn't like it, it's not my type of book." Okay, that's not the author's fault, that's yours for buying something you shouldnt have.
Another one, and I've had this happen to me, is make sense with the review. I have one (I won't say where b/c I really don't want to get this poor person beat down by trolls) that I think the person may have been drunk when they wrote it. They start off talking about how they loved it and couldn't put it down, then flipped talking about not caring for the characters and story....but couldn't wait to read the next book. I've had several other people read that review incase maybe I was misreading it. I was not.
In short, review stuff, be honest, be to the point, don't say crazy things and remember this is for the sake of other people: the author and the potential customers. Thanks, eat cake, be happy.