Writing Groups: Help or Hell?

Have you ever been in a writing group, or thought about joining or starting one? This post about it got me thinking. I was in a writing group years ago, and it fizzled (first for me; then eventually for them). My fizzle was my fault, I'd say, for:

a) not having enough current writing to make it worth going, and worth the group's time to critique me;

b) not realizing that there's limited value in having critiques from people whose writing expertise is based in entirely different markets or styles of writing (unless they are very eclectic readers and/or very astute critiquers), and

c) not being willing to commit the time to prepare pieces that are polished enough to benefit from critique and set aside time for the meeting itself.

And although I didn't find competitiveness an issue in that group, I think that also has to be part of the process when deciding whether to join a group or figuring out membership if you're starting one.

It seems that while writers long to break the isolation and yearn for meaty feedback from people who aren't our "bosses" (our agents, our editors, or others for whom we work) or our friends/family (who may be biased and/or tired of hearing about it), finding people you can learn from, and who can learn from you, is not an easy thing.

If you've ever participated in a writing group that worked well for you, what do you think made it succeed? And if you've been in groups that tanked or weren't at all helpful, why?