I've taken many classes (in college) and workshops through a local foundation, and have had good experiences. Being required to turn in 3 poems per week or one story per week made me produce more than I thought I'd be able to. I like writing from prompts because I then come up with something that surprises me.
I have heard stories about classes or groups ruined by people with ridiculous egos or attention seeking commentaries, but have been lucky, and experienced little of that myself.