I'm seriously addicted to this stuff.Especially for cheap one or two day weekend rentals. I can get a "like-new" car, 5,750 united miles, and put on 3-500 miles for around $25. Instead of having my girlfriend take a bus or amtrak to visit me, I can just go pick her up instead as my personal car is unfortunately (2002) a bit dated.
And with Presidents Circle, I can also pick my car with Hertz. This stuff is addicting and much more exciting then mileage runs, since I earn 50x more miles this way.