I've read quite a few comments lately about whether or not writers should review other writers work; particularly in the Indie industry. Obviously there's no cheap yes or no answer to this question because some people, writers or otherwise, just don't write reviews - ever.
If, however, you are the kind of person who will write a review for a book then I don't believe your profession should enter into the equation. Writers, or at least in my opinion good writers, should be first and foremost readers. That means that it's perfectly ok to write a review, good or bad.
Some people have said they couldn't write a review just because the book wasn't to their personal taste but my question is why did you read it in the first place? I've bought and borrowed many books that I thought would appeal to me only to find that aspects of the book weren't to my liking. If I review that book, even if I give it a 1 star review I should be able to tell people what I personally didn't like about the book and why.
I've only really started reviewing books since my own novels were published - because I know how much they mean to the author but that doesn't mean I should sugar coat it. A book may be very well written technically but have aspects to it that I personally didn't like - if I say that in a review another potential reader might just think 'wow I'd really enjoy that'.
If you're a reader than you're already a reviewer whether or not you make those reviews public. How many of us have ever discussed a book with friends or family and NOT commented on the things we liked or disliked? Probably none of us. Just becoming an author yourself doesn't change the fact that you have opinions, valid opinions and you have the right to share. Even if the book was so bad you couldn't finish it don't be afraid to say that - potential readers want to know and writers simply have to learn to take criticism, deserved or otherwise.
So readers of the world write your reviews without fear just be honest.