Realize this, I beg you. When someone asks you for your “honest opinion” about something, odds are they don’t really want your “honest opinion” unless it means you agree with them and are willing to stroke their ego. If not? Well, then what they really want is you to lie and make them feel good by doing so.
Authors are notorious for this sort of thing. If you are not willing to tell them that their latest manuscript, or even published book, is “five-star” quality, hold your tongue unless you are willing to be cursed at and argued with until the cows come home.