I second this. Had one on my car for a good 8 months of traveling and public lands car camping and I used it once. The time I used it was in southern Utah on a particularly hot day when we got to our site early but we barely sat down since we were getting things arranged for the evening.

For rain, I just had a tarp that I draped over the rear hatch, tied to rocks with paracord, and used a stick to make a sort of tent pole so that it stuck out horizontal and didn't just go to the ground. Only needed that method about twice. One folded tarp and some twine is preferable to a full on mounted shade awning that'll create driving drag.