When I started teaching and was preparing for my first parent/teacher conferences I was told to sandwich any criticism between positive statements. I have found that this works in day to day life also.
If asked for an honest opinion I would start with a positive statement about the pattern selected, color choices, whatever, then forge on to the constructive criticism, if needed, and then end on another positive note.
I would only do this if I was asked for a critical analysis of someone's work. Otherwise I go along with the, "If you can't say something nice..." rule. You can usually find something positive to say if you just look closely. "I can see you've put a lot of hard work into your project." whatever the project is! :lol: