Form ecogeek: http://www.ecogeek.org/content/view/1861/
Basically the Kiwi plugs into the computer or OBDII port that all cars from 1996 have. Using your cars computer it tells you how efficiently you are driving and call advise you how to drive better. It will tell you MPG and the cost of each trip plus a bunch of other data to make you a better driver. It looks like fun actually.
Heres the official site:http://www.plxkiwi.com/product/
Useful? I think so. Especially if it makes you save more than 10% of your fuel bill through driver behaviour.
The question is does it freak out and shut down when you have no option but to drag the annoying hyundai excel driving P plater that has just pulled up next to you at the lights doof doofing!