Jay Lake has a post up about workshops here and some of the things they do or don't do. I agree with a good bit of what he has to say in terms of structure and how they function and that whether they are good or bad for you is situational. For example, I get a good deal out of the critiques of my work. Perhaps less now than ten years ago, but still quite a bit because my investment in my stories is structured a little bit differently from many folks.
On the other hand, I think he missed completely some of the things that I find most important about a writers group, the things that aren't critique at all. And this may be a distinction between an ongoing writing workshop and a writers group, which seem to me to be two different animals.
So, here are some things besides critique that a good group can do for you:
Brainstorming, both on stories and career.
Share industry gossip.
Writerly support and cheerleading.
Cross introductions to agents, editors, and con folk.
Listening to complaints and brags.
And, most important of all, peer friendships.