being honest is probably a good idea
When I was in the trenches, I hated it when someone told me that my copy wasn't good enough. I really do want to please my clients and bosses, and knowing that one of them isn't happy with something I've done is a real kick in the ass. When they tell it to your face it wakes you up and makes you listen. (Or turns you right off, but that's another problem.)
As bad as that is, though, there's worse.
When someone says they love what you've done, then it turns out they didn't.
They may say great, super, bang on. Or they may quietly smile and nod. Either way, the bad thing is that they don't communicate what they really think and feel about the work until later, when it comes as a shock and causes a lot more lingering ugliness than is necessary. And it's critical that the news that your work is off-track come face to face -- it has real impact that way, and there's all kinds of facial and physical nuances that get conveyed as well.
I've heard of clients who just wanted to be nice to the creatives, who didn't want to be mean. But they ended up frustrating the teams involved, and tended not to be satisfied with the work they ended up with.
I know of bosses who just couldn't be open, who would take the work away and fiddle themselves, thinking it was quicker. But they ended up with pissed off team who left.
I know of a suit who wouldn't be open with creatives, but who would complain about their work to the client. But for some reason that person ended up jumping from agency to agency every 14 to 18 months...
Client or agency, you owe it to yourself to be honest with your partners. And that's what we are. Or at least, what we can be.