I think any reader is entitled to share an opinion on a book, but it's a long way from sharing one's opinion to being paid/solicited for it.
I don't think blogs have achieved the credence that newspaper/magazine reviews have, and because blogs are ubiquitous (anyone can start one), perhaps they will never be equal.
Personally, I have no qualms about posting negative reviews on my blog. In studying creative writing, I learned to critique the work without criticizing the author, to give reasons from the text for my dissatisfaction.
This said, if I don't finish or don't like a book at all, I don't review it.