Oh yes, I love them! I think it really helps alot in deciding to buy something or not, if there are alot of reviews that is. If there is only one or two, I take it with a grain of salt.
Those reviews can backfire on the publisher, especially in light of the power of the internet. An example was a couple of weeks back when the publisher of the book tried to strongarm the owner of the internet website "katie.com", Amazon felt the revenge of the nerds. Within a day, the book titled (ironically) Katie.com got over 1000 bad reviews. The rating of the book plummetted. And eventually Amazon had to "delete" all of the unfavorable ratings.
So as helpful as these reviews are, realize that Amazon has the ability to maniuplate them if they so choose.