I would estimate, conservatively, that two insightful peer criticisms I received in my workshop course last fall saved me two to three months of wrestling with the novel manuscript I plan to use as my MFA thesis. Without external feedback (and not for lack of trying), it takes me about a month between writing and revising to come to clear-minded conclusions about the state of a draft on my own.
Two to three months is a lot of acceleration for the relatively short amount of time it takes to write a feedback letter. That level of progress was only possible, however, because all the writers involved were conscientious about their role in creating and maintaining a respectful and supportive feedback-giving environment.
Writing well and giving good feedback are two different maneuvers; they shouldn’t be functions of the same proverbial muscle. When you write feedback for a writers’ group-type setting, you enter your voice simultaneously into two intimate spaces: one where your words influence the chemistry of the group, and