I've used the cup before. I had good results. However, we were driving pretty slow, about 5 mph. When going a bit faster, the whole car, thus the whole camera, is going to go up and down. Don't go too fast or you risk having the camera rip right off the car.
If you want professional level stabilization, you'll have to get one of those production trucks that has a stabilization arm on it. It's sort of like a giant steadycam but instead of a person, it's a car. It's kind of like a transformer, really. I just thought of that. I'll definitely check the emblem on the hood next time I see one.
And here's the other thing. If your camera is relatively stable (tripod, bag, whatever), the bumps may be noticeable but they are acceptable. I was in a taxi in India and I put the tripod on the floor and back seat. I pulled down onthe tripod to make sure it didn't jump off the floor when the car hit bumps. And there were plenty of bumps because there is no such thing as a smooth road in India.
When playing the footage back, it looks pretty good. When playing it back at slow speed or time lapse, it looks great.
So, you can call your local equipment rental company as ask for a grip package for a car mount. You'll get a bunch of bars and clamps and you can mount the camera anywhere on that car your imagination takes you.
http://www.picturehire.com/grip.html
http://www.filmcastentertainment.blogspot.com/2009/05/grip-grab-grog.html
-trevor ward
Red Eye Film Co.
www.redeyefilmco.com
orlando, fl