Worst use of 5 karma to learn a spell ever, or best use. You decide 
I guess it would depend on what kind of personality your character has, how you play them, and what kind of magic you're focusing on. A healer type probably wouldn't see much use for it, while an illusionist/trickster type who likes to play mind games would probably make great use of it. Given the right type of character with enough karma, and the right campaign setting, it could be a LOT of fun. *cackles gleefully*
Oh, now you've gone and given me ideas! What'd ya do that for?