In "But the focus group loved it" marketing maven Seth Godin started a nice conversation on focus groups.
He says the purpose of a focus group is to help you focus.
"Not to find out if an idea is any good. Not to get the data you need to sell your boss on an idea."The best ones I've been in helped put a voice and a face to trends that market research was pointing indicating.. The worst have been where the group was asked open ended questions about what they wanted or would like.
The trackbacks on the Seth's post are good reading. It's almost like a focus group on focus groups.