Yes I have one.... But they rarely work on UK cars.
My understanding is that US cars have to be OBD II compliant (on board diagnostic) so if you're in the US, you can plug in a portable reader and tell what the fault is and, if necessary, clear the code. Or take it to the dealer telling them that an oxygen sensor has failed (or whatever).
However UK cars generally need some kind of specific software between the reader and the car. I had a US imported car, so it worked fine - but I've yet to plug mine into a UK car and see it work.
