Writing groups, in my personal experience, are kind of like bears and pig-tailed burglars.
Being in the wrong writing group kind of really sucks. It's too hot. Competing egos and no real desire to improve the craft have a detrimental effect to everyone involved. Slamming everyone's work in order to make yourself feel more competent is counter-productive and unprofessional. On the flip side, gushing about everyone's work because you don't want to offend can do the same thing.
Of course, being a solitary writer kind of sucks. Too cold. You can't work in a vacuum. There are aspects of writing that just need an outside eye. Grammar people are valuable. Nice grammar people are priceless. Human encyclopedias, style watchers, semi-professional readers, industry gurus--anyone and everyone, all are people that you don't have when you're alone. Duh, I know.
Being in the right one is awesome. Warm, porridge-y, awesomeness. :D Having different styles, voices, viewpoints and goals is the whole point of being part of a group. But the ability to maintain your own and validate those of the people around you while everyone produces at their own pace is so much fun.
So, yeah, I love my writing group. :)