The internet has made it possible for anyone to become a reviewer, and like all developments, this one has both advantages and downsides. I'm very clear in my own mind, however, that the benefits far outweigh the drawbacks. It's good for a very wide range of people to be able to articulate their views, and it's also (usually) to the advantage of those whose work becomes the subject of debate.
Of course, there is always a risk that reviews will be written in bad faith (which is very rare, but does happen) or without a good deal of thought (more common.) But if one writes a book, or a song, or paints a picture, and allows the end product to be made available to the public, one has to accept that not everyone will care for it. Bad reviews can be very hurtful, but there is no escaping them. And the more successful a creative artist is, the more frequent bad reviews are - a paradox, but true, I think.
On the other hand, a good review is heartwarming, and may help a great deal with motivation. Sometimes, writers and other creative artists need a boost to keep them going, and a positive critical reaction can help. So, when I am writing a review, I do my best to maintain a balanced approach, and try to understand what the author was trying to achieve. Sometimes, of course, it's not entirely easy to figure out the answer!
I've also found occasionally, after taking a second look at a book or a film, that my original and immediate reaction seemed to have missed something. Sometimes you see more clearly the second time around.
As a novelist, I've found that one of the most positive experiences is when one's work is reviwed by someone who "gets" what you were trying to do. My first couple of books enjoyed, luckily enough, a lot of positive attention (though one review was one of the worst I've ever had.) But perhaps the best moment was reading a long review, in the New Law Journal, by Frances Fyfield, who had clearly figured out what was in my mind when producing the book, and appreciated it. I'll never forget the pleasure this gave me. Years later, I had the opportunity to say thanks in person when I finally got to meet Frances.
From time to time, I come across other reviews that are equally pleasing. A recent online example is this review of The Arsenic Labyrinth, by a blogger who was prepared to put up with the gradual build-up of suspense (an approach I thought about quite carefully when working on the book) and was pleased to have kept faith in the story. This kind of reaction is good for morale, and there's no doubt that, for any writer, maintaining morale is extremely important. I was at a writers' meeting recently when someone made mention of a vote of no confidence. As the chairman (a very successful novelist whose work has been filmed and televised) wryly remarked, writers are constantly giving themselves votes of no confidence!