There’s an interesting (but obviously not unbiased) post from The Guardian Book Blog called The Literary Universe is Bigger in the Blogosphere that discusses online reviewing.
“But why should we believe the blogger?” comes the cry. “Who are they and how are they qualified to tell us what to read?” The answer is: you should believe them and trust them in exactly the same way you would a critic in a newspaper or literary journal. There will be some you admire and some you think are stupid. Some bloggers write well and some badly and so do some literary critics.
Amen. (And yes, I am also biased. :))
Also: Does it matter if the reviewer gets free review copies from the publisher? (We do.) Or advertising dollars? This information can surely help you decide whether to trust reviews, but ultimately I think you either trust a reviewer’s integrity or you don’t.
Do you trust online book reviews?