Although I'm not comfortable reviewing another author's work, I do like reading reviews on Amazon. I typically wait until I'm done with the book before looking at the reviews. It's interesting to see how people can have such wildly different opinions of the same book--and interesting to see why people give books a low number of stars. It doesn't help anyone to downgrade a book for being a different genre than what you expected or for the cost. I seldom see comments about the writer's craft; readers tend to focus more on the characters, story, and ending.
Do you write book reviews on Amazon? Do you find them helpful?