The question of reviews is of course broader than just Amazon.com. Anywhere you go on the Internet to make a purchase you're going to run into "customer generated" reviews. The reliability of these is (in general) unknown.I have developed a couple of rules of thumb which I have found to be helpful. First, go to the non-Kindle page for the book at Amazon.com -- there will often be "editorial" reviews from Publishers Weekly and Booklist. These are written by publishing industry "insiders", and are not necessarily objective, but they are a good first place to get an impression of a book. Next I turn to the customer reviews (still sticking with the printed version of the book). I tend to give greater credence to books with a lot of reviews than books with only one or two reviews. I tend to read three or four of the most positive reviews, and one or two of the most negative reviews; and from this and make a general purchasing decision. Then I go back to the Kindle page, and see if there are any reviews there (particularly negative ones) -- and check that there are no complaints about the formatting or structuring of the Kindle edition. There are situations where Amazon will sell you many titles as a single book (these are usually older, out of copyright, public domain books). It is important to check (if you can) if there are any problems with how the multiple titles are included in the single book -- many times you can be 10 books in to a 30 book series, want to go back to the second title in the series, and if there is not a good table of contents (often there is not), this can be difficult to do. By and large, I personally, know pretty much what I like in a book and don't need to rely much on reviews -- I have my favorite authors, and genres, and pretty much stick to them with few exceptions. However, when I am buying something more tangible from Amazon (or any other e-commerce site for that matter), I find that customer reviews (using the protocols described above) can be quite helpful.
This is been a bit long-winded, but I hope it helps.