I have had a guy confess his love for me in a speech once. His words were very powerful...and even shocking to me, because nobody had ever used the words he did to describe me before. He of course waited until it was too late and treated me like a burden the entire time we knew each other.
So, I think a speech is good if you know the girl likes you and it is not too late. If the girl does not like you, all you can do is hope she has enough class to not gossip bad things to her friends about you.