Quote Originally Posted by Failte View Post
! Вы очень полезны!
I think it's better not to say that way. We usually say полезный only about thing, very seldom about people. We could say for example он очень полезный человек or полезный специалист, but it's unusual, and I wouldn't say that myself. If you say Вы очень полезны, it sounds like you use the person like a thing, and I don't think the person would be pleased with it. I think the most common expression in cases like that is Вы мне очень помогли.