I've just got cheap carpet in the cargo area of the wife's Prius. I really liked the fancy floor matts, but add in the extra large one for the way-back, and whoooo pricey!
At some point, I would like to upgrade to a little larger piece of carpet and match the interior color better. I also plan to run the carpet up the back of the back seat (and put in a slit for the 60/40 split seat) so that the carpet also covers the back of the rear seats when flipped down.
There's something the factory matts don't do!