I have a few thoughts:
Reviews in general are open to abuse, and this seems to happen a lot on amazon. We're all aware that some authors will get their friends to write glowing reviews, and in fact it's been well publicised recently that some authors have made tons of false accounts, and have not only given themselves 5* reviews, but have also slated their 'rivals' with 1* ones. There are also people who can be paid to write 5* reviews!
As a reviewer myself this is making me quite sad. My reviews tend to be positive, because I choose books to review which I think I will enjoy. I have given lower star reviews, but mine are often 4* or 5*. I do wonder though whether people believe what I'm writing, or if they think I've been somehow influenced by the author.
Then we have the recent issue, highlighted by the JK Rowling book, where reviews have nothing to do with the book. There are both 5* and 1* reviews listed which have nothing to do with the book, only the price. I've also seen this happen for other things.
This is just annoying, and I think it should be better policed by amazon.
Finally, there are the reviews which manage to give away the whole plot, including spoilers. I tend to glance at a review, and if I see any details, I immediately move on!
Amazon do have a facility to show if you've actually bought a product from them (I think it's listed as a verified purchase). Whilst I appreciate that this may make the review seem more dependable, I don't think the others should be discounted. Amazon is still a place where people go for book reviews, so it should be a good resource. I may buy a book elsewhere, or be sent it by the publisher, but I still like to put my review on amazon for others to read.