I can't speak to the rags, but I do love Oil Eater - very cheap and really effective. I use it on the driveway since GM's choice in locating oil filters makes it impossible not to drip a bit (hey, it couldn't be my technique!) - and there's always something to clean.
I've also got Poorboy's Bio APC, and I really like this product for tires, wheel wells, and general cleanup.