I have a Verizon Wireless Motorola phone with one built in, and it's amazingly accurate. It's actually one of the cheaper phones, not some snazzy gizmo that costs a lot. It will upbraid me if I pull over for a Coke and try to get me back on track.

I don't have to pay for another gadget, and I carry it with me no mater whose car I'm in. I heartily recommend it.