I think you should be honest with the bride, she might be thinking the same thing as you, but not want to say the dress isn't flattering on you to spare your feelings. If you tell her you feel very uncomfortable wearing that style, but if that is what she wants then you will wear it, she may change her mind about the style. I would definately want my bridesmaids to tell me if they hated the dress, especially if I was having them pay for the dress. Be honest with her, her heart might not be as set on the dress as you think it is.