I've participated in a number of group critiquing experiences and find them all to be somewhat lacking in efficacy, for different reasons. Primary among these is that each of them has suffered from competitive subtext, so that one could never be certain whether or not the critiques one received were motivated by a genuine desire to be helpful. Therefore, I don't usually recommend them to others, since a good degree in any subject from a reputable college or university, an earnest level of investment in the study and practice of the writing craft and a tenacious commitment to selling ones work suffice for most people interested in writing professionally.
However, I did recently investigate two writing groups because I'd heard good things about them from reputable sources. The first was the Online Writing Workshop, and the second was the Codex Writers' Group. I didn't intend to actually participate in OWW; I just wanted to see what they were all about. I had an interest in Codex because a writer I respect said good things about the group in a recent interview. I have allowed my trial membership in the first to expire, and as of today the second has not responded to my membership inquiry, so I have little to say about those two resources. Still, I will write what I know about them as I review those experiences and workshops with which I have more knowledge.
The Master's-Level Creative Writing Class
I took two creative writing workshop classes while working on my MA in English. In both of them, the focus was on literary fiction and poetry. In my case, I was fortunate enough to work with professors who had no bias against genre fiction, but my fellow students often didn't have the skills to critique it, and for that matter, neither did one of those professors.
Frankly, you must read a great deal of genre writing before you can point out flaws in genre fiction. This means that most people working on creative writing degrees can't address issues like over-used tropes, bad science or failing to adhere to the laws you've set down for magic in your fantasy worlds. In addition, many creative writing programs, especially at the master's level are still biased against genre fiction, even when they say they aren't. So you might find yourself in a place where nobody knows how to critique your work, and your professors hope you'll eventually give up your genre habit for something more 'literary'.
Moreover, this is probably the worst place for the aforementioned competitive subtext, and it's sadly the place where it makes the least sense. In my experience, most people working on their MA degrees just aren't writing as well or as often as peers outside the classroom who are paying their dues in genre fiction. So ultimately, there are far better options for genre writing instruction than a creative writing degree. You'd be better served with a degree in Physics.
On the whole, the Critters workshop was a good experience for me, but not for the reasons you might expect. I think I workshopped maybe two stories during the three years (2001-2003, if memory serves) I participated in the group, and both of them received mostly useless critiques from people who were either not very skilled writers themselves or who responded to my work with that peculiar variety of snark practiced by people sitting anonymously behind their keyboards.
However, it was an invaluable place to discover what worked and did not work for me in the writing of others. It was also invaluable to watch my ideas lifted whole-cloth from stories I submitted for critique and situated in somebody else's work. Now, by this I do not mean that my writing was so brilliant that people wanted to copy it. What I do mean is that when you're thinking about writing a story, often the first things that come to your mind are things you've seen before. So I learned not to release any idea for critique that I didn't want to see dropped into the collective idea pot.
I stopped participating in the Critters online workshop back in 2003, when it became too much like grading papers for me. However, I would recommend it to new writers looking to hone their craft with the following caveats: Don't submit any of your own stories for critique unless they're experiments in writing you have no desire to publish, and do critique at least two stories a week written by your fellow critters.
Clarion and Other Milford-Style Workshops
I attended the Clarion workshop in 2006. Two years later, I still can't decide whether it was more helpful to me than not. On the up side, it places students in close contact with established masters. On the down side, sometimes those established masters are, in the words of a well-known writer I deeply respect but who shall remain nameless, "bug fuck from the time they get up in the morning until the time they go to bed at night." On the up side, you have the opportunity to do nothing but write in the company of your peers for six whole weeks. On the down side, you're stuck in a closed group for a month and a half with people you've never met in a situation rife with competitive subtext. On the up side, you learn a lot about the financial and social mechanics of the writing industry. On the down side, you sometimes learn it the hard way, from bug fuck masters and the little social climbers among your peers.
And ultimately, when you think about it, there are three Clarion workshops turning out fifteen students on average per annum, not to mention students attending other writing workshops like them. This means there are at least fifty people every year being taught the same skills and techniques you are. They're your competition for publishing slots afterward, and they all sound, to some degree or another, just like you.
So IF you're willing to accept that your workshop instructors might or might not be helpful to you and indeed might be harmful to you, and IF you can keep your head in a competitive situation where scapegoating is commonly practiced, and IF you are willing to accept the unfortunate truth that writing is cutthroat and your peers have the sharpest knives in the house, Clarion CAN be a SOMEWHAT constructive experience. However, I would strongly urge you not to talk about the details of any major project you might be working on while there. Keep that stuff to yourself. And try not to write like your fellow students while attending the workshop, or when you come home, or ever.
Online Writing Workshop
Again, I was only a member of this workshop for a month, and I only visited the members'-only areas of its web site one time. While there, I read the first page or so of a few stories up for critique and saw much the same level of skill I did while participating in the Critters workshop. OWW does boast a rotating panel of established writers at its helm, and it does appear to have a more robust interface than Critters. It also boasts a few well-known writers as former and current members. From that information, I gather it's probably a better place for people interested in workshops. However, I would venture to guess the same caveats listed above for Critters also apply here.
Codex Writers' Group
This group occupies an interesting middle ground for writers. To join, you must either be a graduate of an audition-only, Milford-style workshop like Clarion or you must have sold three stories to markets considered professional by the Science Fiction Writers of America. Its purpose appears to be the facilitation of critiquing and networking for writers who have just begun to produce publishable work, and it has been favorably mentioned by a well-known writer in a recent issue of Locus. In the interest of full disclosure, I should mention that I'm keener on the networking side of things than the critiquing side of things, if only because of the aforementioned idea-pool problem. However, since I am not currently a member, I can't write about either side of the group's activities with any greater detail than I have given here.
So it's clear I'm not excited about writing workshops. Ultimately, I doubt they help writers as much as reading helps writers. Once you know how to pick out what works and doesn't work in a story, not only do you become better at critiquing your own work, you also become better at learning from good writers already in print. In terms of the industry knowledge imparted by instructors at workshops like Clarion, well, that's what an agent is for. If I had any advice to offer then, I would tell you to read a lot, a lot more than you already are. Get a good degree in something. Study the industry. Write every day that you eat. Market like a twenty-dollar whore on a Sunday morning. Workshop only if you must.