My version isn't cursed, per se. It's just an item with both good and bad points. Free use of a 9th level spell on a daily basis is no small thing. Even for high level casters, it frees up a 9th level slot that can be used for something else, since Time Stop is one of those "Gotta have it" spells.
And I have to say that I disagree with the ida that cursed items are just there to make PCs miserable. Properly used, they become plot hooks. If nothing else, they keep the game world from turning into candy-land, they keep the players on their toes.
Do you "identify" a magic ring by just popping it on your finger to see what it does? Watched a guy do that at a recent convention game. He didn't think anything of it, and was surprised when the other players objected. And yes, it was a mixed blessing kind of cursed item, and he paid for it.
No, literally, we made his character pay for the item, since he'd taken it from party treasure and couldn't return it.
The game has a number of items, feats and PRCs that fall into the "mixed blessing" category. Some are artifacts, some aren't but all add to the richness and depth of the game.