angels, souls and vessels (oh my)

So far we've been told/shown that angels can only enter a body with that person's consent. In S4's The Rapture, Castiel left Jimmy when he was pulled back to heaven; he returned by entering Jimmy's daughter (I guess in this instance, children can give consent? Or do the rules differ for kids? Guesses, anyone?) At the end, when Jimmy's dying and begging Castiel to come back into him, Castiel tells him it's forever, then, no going back. (Right? I didn't misunderstand that?)

So, does that mean Jimmy is essentially *gone*--only the shell remains? Is Jimmy's soul still in there, somewhere, presumably asleep if it is? Or is he truly an empty vessel, filled only by Castiel, and that part that makes Jimmy Jimmy, the soul, is it gone onto wherever in the Supernatural universe souls go when a person dies?

I'm trying to figure out, at the end of 5x04 ( and I'll cut here for the whole spoiler thing )

Thoughts and speculations more than welcome :) (And if you've not seen up through 5x04 and don't want to be spoiled, probably shouldn't click, as there will undoubtedly be spoilers.)