The passive bonuses on Moon Stone are good. But how to play it?
There are two main ways:
Specific card effects. The best are those printed on investigator cards that you start the game with: Wendy Adams, "Ashcan" Pete and Patrice Hathaway. The best non-investigator way to discard at will (there aren’t many) is Scroll of Prophecies.
Discarding cards due to maximum hand size. Anyone can do this, but it could be difficult if your hand tends to run low. So other than the investigators listed above, Moon Stone would probably only be used by investigators that really want the willpower and agility bonus and have no better or more reliable accessory options.