We know a little about a lot of things; just enough to make us dangerous. I can dig Elvis. There ain't no me if there ain't no you.

In Castiel's final moments, he smiles through his tears as the Empty takes both him and Billie, saving Dean. Castiel's final words are "Goodbye, Dean." I have to say, I had a gut feeling Castiel wouldn't make it out of the series alive, and as devastating as that was to watch, what a way to go.