Two thoughts come to mind:
1] if you do these too often, people will get burned out and WUMP will die. It's probably a good idea to set up a schedule, maybe the first of every month or the first and the fifteenth of every month (if you want to do two/month), so even if people aren't here every day they'll know when the next one starts.
2] if people aren't taking the constructive criticisms to heart and further examining their work, but instead getting their feelings hurt and spending time being defensive, they may as well not participate in the first place. This is a learning experience for the participants, if you go into it thinking otherwise, you're missing out...
Just some thoughts.