One of the cardinal rules of group brainstorming sessions is, "not criticism!" The idea is that you note every idea and then analyze them in a separate session.
However, a study at the University of California, Berkeley, found that groups in which criticism was allowed came up with more creative ideas than a similar no-criticism one.
My guess is that this can work if the criticism is brief, constructive, and when the session is led by someone who makes sure the group gets back on track quickly.
If you were to conduct such a session, it might be useful to ask the criticism to take the form of how the idea could be made better rather than why it couldn't work.
