Writers help each other out (or they SHOULD). And one of the biggest services is supplying critiques. But if you aren't familiar with best practices, you could end up doing more harm than good to your writing buddies. So let's review how to review a manuscript from start to finish, shall we?
It takes bravery to let outsiders handle your writing. However, the feedback you receive is valuable. One of the best resources writers have available is workshops. You get the feedback from a number of people with different backgrounds in one place (some more useful than others, but that's the risk you take).