How honest should you be? You know the famous question: does this dress make me look fat? There are several ways 
to answer, none of which will end well. If a new spouse makes dinner and asks whether you like the meatloaf, there is only one answer: "hmmm, I love it." If you say there are too many onions, that's trouble. If you say nothing and your spouse later learns you've been force-fed onions for three decades, that's big trouble.
Your boss says, "read this and tell me what you think." If it stinks, how do you answer? If you're asked whether the ridiculous, new, hair-brained sales scheme will work, do you tell the truth?
I think you should, yes. Tactfully, clearly, quickly, tell the truth.
And if someone tells you the truth, about your meatloaf or your sales presentation, see it for what it is, an honest attempt to make you better. It's probably not a personal attack.
