My answer to this question is a cautious sometimes.  A group’s value depends upon what you personally need and, even more heavily, upon where you are in your writer’s journey.  I had the good fortune to be a part of two strong writers group early in my own journey and found them very helpful.  For a writers group to function, it must be able to combine honesty with kindness.  If members are unkind, spirits will be broken.  If members aren’t honest, writing won’t improve.  Additionally, everyone must carry their own weight.  If some members only give cursory reads while others commit hours of effort, there will be an imbalance in participation which, along with dishonesty and unkindness, can become a breeding ground for angst and resentment.

In my early years of writing, I had a pretty good concept of story and character.  I was also developing a sense of scene and sequence.  But there were many finer points of the craft that I still wielded like blunt instruments.  I suspect this is true for most burgeoning authors.  In my first group, I was lucky to have a member who was very skilled with grammar and called me out whenever I got sloppy.  I also had members who were willing to tell me when my descriptions were dull or my beloved characters were behaving like jerks when I thought they were acting gallant.

Giving and taking feedback is, of course, an art as well.  When I received these critiques, my first instinct was to bristle and tense up.  But I allowed myself time to process their advice and I usually ended up agreeing with them.  In my first attempt at a novel, I eked out almost 40,000 words before I realized I was going in completely the wrong direction.  I scrapped that plan, went back to the drawing board, and started it all over from the beginning.  That novel is done now, and is much better than it would have been if I’d ignored that feedback.  That being said, it was a “learning novel” and still needs lots of attention.  Maybe one day I’ll start it all over again and bring it up to a standard that makes me happy.  If not, I’m still wiser for the experience.

You can also learn a lot through editing someone else’s work.  You can see what you like and what you dislike about what they’ve done.  You can understand their process and adopt techniques that work well for you.  And, frankly, you can get better at editing your own work.

So writers groups are great, right?
Not necessarily.  Writers groups, even excellent ones, have their problems.  The biggest shortcoming is that a writers group may shackle your writing tempo.  Unless each of you has a ton of time on your hands, writers groups must, of necessity, take turns.  In both groups I was a part of, we met once a month.  There were usually five or six members and we looked at two or three contributions each session.  That meant I usually got a turn every second or third month.  Generally each person was capped at approximately 5,000 words for their submission.  Thus, if I’m working on a 100,000 word novel, it will take me twenty turns.  In other words, to share the whole book once, it will take almost five years (20 turns x 2.5 months = 50 months).  And that’s not considering re-writes!  Add those in and you may be looking at a few more years of work.  It may take the greater part of a decade to get through a single novel.

Along the same vein, if you set your own work around your writers group’s feedback schedule, you may be hamstringing yourself.  By only putting forward 5,000 pages every couple months, you may find it impossible to hit a stride as you craft your story.  If you dare to forge on ahead, you’ve left your critiquers in the dust.  Will they be able to give meaningful feedback reading Chapter 1, then skipping the next four and picking up Chapter 5 two months later?

Additionally, taking bite-sized chunks of a book every few months tends to lead to a certain kind of feedback:  a highly focused line-by-line, paragraph-by-paragraph inspection.  This can be very helpful for new writers still figuring out their voice.  I know I found it useful for a time.  It can result in really good sentences and paragraphs.  But working at such a detailed level with a half-dozen scrutinizers may result in overdone reworks.  Your knife-like prose may be sharpened down to a nub.  There also is the problem that that this level of feedback will often fail at higher level concepts such as pacing, character arcs, and any blind spots you’ve unwittingly created.  Most people don’t have the ability to retain enough of a grasp on a story when reviewing intermittently.  You’ll end up stuck in the weeds.

This leads to my final conclusion.  A writers group can be a great tool and source of support early on.  It can hone the rough edges of your writing.  It certainly did for me.  But at some point, once you have sufficient confidence in your own writing skills, you will probably need to move beyond the restrictions of a group and fly onward without them.  I say this with two caveats.

First, I’m making assumptions about the nature of the writers group as per my description above.  A group which can handle larger pieces of material, for example, may have a longer lifespan.  Second, if you do move onward, that doesn’t mean you have to move onward alone.  It doesn’t even mean you have to abandon your contacts in the writers group.  That’s what happened to me.  Though I left behind the group format years ago, I’ve maintained contact with two other writers that I met there.  I still talk to each of them one-on-one about my stories (and they do the same with theirs).  They assist with brainstorming during the initial planning stages, they help troubleshoot hurdles as I come across them, and from time to time, I dump a quarter, half, or even a full novel on them.  It may take them a month or three to read through it, but they graciously do so.  Reading that volume of material as a single project allows them to see far more broadly than if they’d read it piece by piece.  Obviously, don’t take advantage of such relationships.  Just like in a writers group, there is give and take.  But at this point, you won’t be grinding over every little phrase.  Rather, your craft will be strong enough that you’ll be seeking feedback on those high level concepts that can make your story truly great.