I got a $9 from ebay, aus seller so was with me in a couple of days, one of the little blue ones you see on there.
Works a treat, use it with Torque and OBD Car Dr (prefer there graphs). Havent actually found a reason to have one, my excuse for getting one is if im out somewhere remote and get an engine light come on, i can hopefully diagnose if im safe to drive out or if its something serious.
Its only a few dollars and its a cool little toy to have.