This is a hard question, because it varies from book to book. If you're confident of your work, you can't let every niggling comment knock you down. When I write, I can sometimes still hear backbiting from workshops I've attended, and I have to tune it out, which isn't always easy. On the other hand, when I know that someone is a good reader for a particular book, I have to take what she says more seriously than if I knew she wouldn't pick up a book like that except for my having written it.
In other words, it may take a little matchmaking to find the right reader.
But first and last, you are always the most important reader of your own work. I like to read the book aloud to myself and see how it sounds. If it sounds canned in places, or I find my mind wandering, I make a note of that and keep going. Often, I realize that the rough spots are holdovers from previous drafts that took on lives of their own and may be revised out--or, contrarily, they were shorthand for something I meant to flesh out but didn't, and now's the time to fix that. Every blind alley or false start jumps out at me when I read aloud, and if an image sounds lame, I can hear it. If I've overused a word or phrase, I know it right away. I don't go looking for things to make special, but if it strikes me the wrong way, I'll work on it.