As is known, criticism is useful, but so is the way you give it. Cause as a rule, people are vulnerable, so they begin to defend themselves at once, missing the chance to listen to the end and make an appropriate conclusion or change for the better.
Personally I like when people give me honest and constructive criticism. Oftentimes I cannot see all my faults as they are seen from the outside, so I am grateful to people who help me see them, thus giving a chance to perfect myself.
That's not to say that we should find faults with everybody, no! But your friends are worthy that you share your honest opinion about whatever! They say a true friend is one who says the truth whatever happens.
No doubt, you should possess the art of diplomacy to be able not to offend a person, but open his eyes to something. So, first of all you should frame your criticism in appropriate language. Don't be rough, better start your comments with "It seems to me…," or "I could be wrong,