[Nils]

I thought that the Nine would just give Power, without any visible symptoms, but slowly turn the user into a wraith (how fast depended on the sincerity of the users' original intentions)

From Of The Rings Of Power And The Third Age:

Men proved easier to ensnare. Those who used the Nine Rings became mighty in their day, kings, sorcerers, and warriors of old. They obtained glory and great wealth, yet it turned to their undoing.They had, as it seemed, unending life, yet life became unendurable to them. They could walk, if they would, unseen by the eyes in this world beneath the sun