If you use Facebook there's lots of writer groups on there. People read each other's stuff and if you're really lucky someone will tear it apart for you. You'll need a pretty thick skin though if you're going to learn anything from it.
Reviewers on Amazon, etc, unless they are other writers, don't usually tell you anything useful. It's ether love it or hate it with them, neither of which helps with improving your story. And reviews are forever, even if you do fix whatever was wrong with it.