If someone asks for your honest opinion, and they're looking for a critique, yes, but if you go to your kid's art show at their junior high, you wouldn't walk up unprompted and say Not your best work, son.

Constructive criticism is an art, and there is an appropriate time for it, but there are plenty of people in the world who are very happy to offer their unvarnished opinions, and very few who are going to be a cheerleader for you. People need cheerleaders as much as they need honest opinions. Knowing which is needed in any particular situation, is also an art, but I think that most of the time, finding something nice to say is almost always the better path, simply because so many others are choosing the other one.