To me, the purpose of a review is to find out whether or not I will like the book. In order to do that, there has to be some substance.
If a reviewer says: this book is great! 5 Stars!!! That doesn't help me in the least, and I skip over it.
If the review says: This is what the book is about, this is what the author did right, and this is what the author did wrong. I will pay attention to it.
I guess the shill reviews are more about getting quantity than anything else, right? In that case, it doesn't really bother me that much.