Demons we know can and do, in the SPN verse, inhabit and animate a dead (deserted by soul, spirit, personality) body. They can also share space with a living person, heal injury well enough to keep the body functioning until the demon vacates, at least, but observation suggests the healing effect leaves when the demon does.

Angels appear to burn away the physical, spiritual and mental elements of a host. But it seems they're able to heal almost as fast as they consume--almost being the key. The longer they inhabit, the greater attrition on the body.

The Winchesters, through John's bloodline, have physical bodies better able to physically withstand angelic containment demands, theoretically for sustained periods. Jimmy was apparently with Castiel, his personality and spirit unharmed, through careful management by Castiel. Still, his experience was "like riding a comet". I think Lucifer would have little regard for sam's spirit and no use for his personality, and would take little or no care to keep it from burning away. He would consider the vessel property, rather than a loan.

I don't know about Michael. From what we've seen I think he's as cold and arrogant as Lucifer. He might make provision to cushion his vessel's consciousness, but he would consider it a minor courtesy, and not mourn overmuch if Adam dies from his duty as vessel--he was raised from the dead, after all, by the will of heaven, and to heaven he will return, a reward for his service. What's the problem?

Angels appear to burn away the physical, spiritual and mental elements of a host.

It depends on the power of the angel, according to Castiel. So he might not have done anything purposeful to save Jimmy or Clare. And Michael promised not to do that to Dean. So it's variable.

Jimmy was apparently with Castiel, his personality and spirit unharmed, through careful management by Castiel.

The key being care. Raphael took little or no care of his vessel, leaving a drooling hulk behind. Anna's, Gabriel's, Uriel's, and the other angels we've seen extinguished while in their vessels, the vessels died as well. Michael has said he'd take care of Dean. Whether he made the same promise to Adam I don't know, or don't remember. Lucifer made no such promise. In fact, he's implied that Sam would be consumed, alhough I think he wouldn't care if Sam was conscious and present for a while, or not. I don't see him planning to relinquish the vessel back to its original occupant, whether it was damaged or not.

What I was thinking was that if Sam's soul wasn't in his body his consciousness wouldn't be either. That his body would be just an empty shell being used by Lucifer

It also occurred to me after I posted that once Sam has allowed Lucifer entry into his body he's probably going to need to have his consciousness present to carry out whatever plan they come up with to trap Lucifer in hell. So unless they have a two part plan - part A, trap Lucifer, part B, get Sam (either all of him, or at the least his soul) back out again - I have no idea how they wiggle out of this one.

edit - The biggest struggle might actually be for Sam to retain his consciousness rather than for it to be submerged by Lucifer.

Where did Lucifer state that Sam would be effed up? The only cite I can find about default vessel status is that the more powerful the angel the worse off the vessel is on extraction (Free To Be You And Me).

I am assuming that Lucifer will mess up Sam, since he's at least on Raphael's level of power. Just wondering why you think that Castiel would mess up Jimmy by default, since he's never said that's the case.

serial:

It also occurred to me after I posted that once Sam has allowed Lucifer entry into his body he's probably going to need to have his consciousness present to carry out whatever plan they come up with to trap Lucifer in hell

And Sam is asking questions about demon possession, not angel possession. I'm not sure why he thinks Meg or YED experiences count.

And Sam is asking questions about demon possession, not angel possession. I'm not sure why he thinks Meg or YED experiences count.

True! Even Lucifer very pointedly told Sam he needed to ask permission because he's an angel. Good point.

Okay, with all this vessel talk, I have a question. Is Jimmy still inside Jimmy along with Castiel? If Castiel is human now, doesn't that make it ethical for him to turn his vessel back over to Jimmy because he's no longer fighting with angel mojo? I just don't remember if Jimmy is already gone.

In theory, I'd have been okay with Jimmy having achieved his eternal rest when Raphael killed Castiel in 4.22. But My Bloody Valentine mussed things up with the vessel cravings. Cravings of the body? Cravings of the soul?

And I'm going with Castiel having a speck of mojo left. Just because. If he doesn't have any mojo left, could he even give the vessel back to Jimmy? I mean, something supernatural would have to be going on for that to be possible.

Jimmy and his vessel died at the end of S4 when he stood with Chuck against the not-identified archangel. Castiel's reconstitution in a shape replicating his former vessel was an act of whomever yanked the Winchesters out of the abbey and put them on a plane. It was assumed at the time to be God. And at the time, we were told that Jimmy was no more. However,

My Bloody Valentine mussed things up with the vessel cravings

So I don't know if Jimmy is actually still in there to take repossession or not.

Anna reconstituted a container in the shape of her former vessel because she liked it and it was familiar. In her case the original owner of the orignial vessel was actually Anna, void of grace.

Lucifer has not stated that Sam would be ruined. He has rather pointedly avoided saying anything about the condition of Sam's personality, identity, or soul, post-claiming. It's my presumption that his arrogance holds everything but Sam's physical ability to contain his Luciferness in abject contempt, and he wouldn't even consider giving it a second thought. Sam must agree to accept Lucifer so Luci can claim his vessel, then Sam is done. The implication being that Sam's soul is burnt up and the atoms scattered, along with everything that makes him Sam.

In The End, the writers were very careful to not have Luci say, "He's in here with me, you brother." There was no Sam, Sam was gone. And that, I think, is a choice made by Lucifer.

Michael made the choice to promise. Whether he honors that promise, or whether the promise is good to anyone but Dean remains to be seen.