Looking at the numbers, even a reasonably optimized character only hits vs AC on an equal level foe about 50% of the time. Against an elite, solo, or higher level monster it can get much lower than that. I'd probably be pretty annoyed by the item too, unless I was an Elven deadly trickster or something. Given how many rounds fights go in 4e, the odds of this item taking the player out of the fight late in any battle are pretty good.
Cursed items aren't neccessarily a bad thing to introduce into the game. I'd just do it very carefully. Alot of thought needs to be put into the mechanical implications of the item, as well as the personality of the player you are giving it to.