I did exactly the same. Not on a regular basis though, but on days when work was tiring I used to head out to the parking lot and sleep in the back of my car. Typically 20 min. That made me much more productive than I should have been w/o the nap.

When one of my co-workers found out, he started borrowing my car keys (he didn't own a car then) to sleep in my car :) - apparently, he thought it was a great idea.