This device allows you to charge two USB powered devices via the cigarette lighter port on your car. This is primarily advertised as an iPhone charger but it will work with any device that can be charged over USB - sat navs etc.
One use I have found for this is not draining your iPhones battery when using one of the sat nav apps. These normally use lots of power so that when you get to your destination your phone is nearly dead - with this it will still be fully charged! If you are a passenger you can also use this to watch videos and play games without draining the battery. There are two ports as well so you can do both at once!
The unit itself is well constructed in black and grey plastic. The connection is quite loose although this shouldnt really matter.
Overall for the price and the convienence of two USB ports you cant really go wrong.