One thing I've learned to ask people is "what kind of critique do you want?", which saves everyone a helluva lot of time and wasted effort. Some people want none at all. Others just want broad "did you like it?' type comments, others want suggestions, and a few brave souls say "take it apart".
I agree that it's great when someone is willing to do that. (The Intrepid crew have been especially unflinching and I think their new efforts are going to reflect that)