I don't know about smelling, but I guess the answer would be easy if you look at Bilbo or Frodo. If they had been wearing the Ring for too long, they'd eventually fade. I assume with the Nazgul, it would have been similar. Gandalf seems to hint as much when he's explaining the Ring-lore to Frodo. Those of them who were kings would remain ruling their kingdoms, people would consider them "blessed with unnaturally long life" and possibly even revere them even more for that (after all, having a powerful immortal ruler feels pretty cool for the people, at least in the "international politics": if you imagine that some of the Nazgul were kings from East or South, where you can imagine lots of wars between small kingdoms and tribes going on, having a strong immortal ruler would definitely be a boost to the nation's morale, and something rather scary for their enemies).
Eventually, however, the Nazgul would succumb to their Rings, fade, became mere shells and enslaved by Sauron's will. Then, I guess, either Sauron would call them to himself (i.e. the king simply one day picks himself up and walks away to Mordor to work on some errands for Sauron that are now more important for him than some small Easterling kingdom) or if he cared, he could make them join their realms to Mordor or whatever. In any case, Nazgul were too powerful to just waste their potential on having them sitting on a throne somewhere far away, Sauron could easily replace them with anybody else if he had the power. Or that's what I would think.
But as for the process of fading, like I said... the best illustration of how it would go is really the part Gandalf mentions to Frodo in The Shadow of the Past:
Quote:
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A mortal, Frodo, who keeps one of the Great Rings, does not die, but he does not grow or obtain more life, he merely continues, until at last every minute is a weariness. And if he often uses the Ring to make himself invisible, he fades: he becomes in the end invisible permanently, and walks in the twilight under the eye of the dark power that rules the Rings. Yes, sooner or later – later, if he is strong or well-meaning to begin with, but neither strength nor good purpose will last – sooner or later the dark power will devour him.
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