I think it borders more on what you would normally say about an idea/item/etc. When someone offered you a deal that does not particularly sound appealing but you do not want to be confrontational, you might simply say that it sounds great to move on and get out of the conversation. If in the environment where you can speak freely, you then might say "to be perfectly honest, this deal is a horrible idea" and not have to worry about the ramifications of those around you because you know your audience.