Quote from the web:
'We barely have time to mourn, though, because we're too impressed by Collins' work in the final scene. He's played so many versions of his character on the show -- Castiel, Jimmy, God, human Castiel, and now, the manic, hysteric Leviathan. If this show wasn't on The CW, we'd be talking about guest-star Emmy buzz".
Hard to disagree with!!