Never had a character who had one. Never that I can recall played in a party with anybody who had one or DM'd for anybody who had one. In other words, in all my years of playing D&D I don't recall ever seeing one in play. However, having fairly recently read the Jack Vance story "Morreion," where they originate (and where fwiw they're consistently spelled in all caps -- IOUN stones) that may change -- not for typical/standard/low-mid level play, but I'm liking the visual of high level wizards with clouds of these things following them around.
(The Vance story, btw, originally appeared in the anthology Flashing Swords #1 (1973 - oop), and was later reprinted in Rhialto the Marvelous (1985 - oop), and the collection Tales of the Dying Earth (2000 - in-print))
(The Vance story, btw, originally appeared in the anthology Flashing Swords #1 (1973 - oop), and was later reprinted in Rhialto the Marvelous (1985 - oop), and the collection Tales of the Dying Earth (2000 - in-print))