Has anyone actually seen any tumbleweed drift by?

Yes, they're literally weeds that end up in a somewhat rigid, reasonably spherical shape, and they're pretty light. I'm in Central California, and I lived in a mostly developed neighborhood that also had a large field that was not developed upon for a while. They would grow in the field, and they don't take much water to grow to be 3-4 feet in diameter; they're well suited to grow in desert areas. When a good gust of wind hits them hard enough, they'll break away from the root, and the wind (or even a decent breeze at that point) will roll them along in a tumbling motion, as they're not truly spherical. That's why they're called "tumble weeds". Anyway, you can easily pick a dead one (brown)up with one hand after it's dried out a bit, but green ones are a good deal heavier because of the water they still contain. For being light, they make a good impact when you hit a large one with your car as it is blown into your path, and they'll knock small children over with a few scratches. It's kind of funny and surreal to see several blowing down the road at the same time, and, as others have said, they love to get caught up in wire and chain link fences.