I think they're a fantastic idea!
I did one that was just 8 friends that all loved to cook, at one of their vacation homes in Big Bear Lake, CA. We all shared recipes, cooked for each other, and learned quite a bit in 3 days' time =0)
I have another friend who takes 2 days each month for cooking classes with her young teen daughter. They spend the day learning together, and I've seen so many wonderful things (cooking and otherwise) come from it.