You might already be thinking, yes, I use this already, or no, it's far too dangerous and probably illegal where I live. Any of those things could very well be true, but a car can be very useful as a DIY dolly. I've utilized cars more times than I can remember just for this purpose, and they work great in a pinch or when a dolly would just be impractical. Even if you already know all of the benefits, there might be some ideas in this Vimeo Video School clip you haven't thought of before:
Safety is unquestionably the most important factor when dealing with any moving vehicles, especially if someone is riding on them in a way not originally intended (like in the video above). It may seem obvious, but if you're ever going to be doing anything where the camera person is not completely strapped in, make sure that the car is moving at walking speeds or a sudden stop could mean serious harm to the person (and possibly to your expensive DSLR).
It's a great idea, and I'm not sure I've ever thought of it before, but putting some in a back seat and keeping the shot tight is brilliant. You can feel free to call me an idiot for not thinking of that, but it's such a simple way to shoot a scene without having to worry about the actors driving the car. So there we have it, one way to make shooting more dangerous, and another to make it safer. I've had good experiences with cars, but again, the key idea is that you don't want your camera person able to fly out of the car with a sudden stop.
We don't condone anyone performing any illegal acts, so be sure to find out the laws in your area (and to what degree you're willing to break them to get a shot).