Well, they work pretty good as long as the disc isn't totally f@#@ED up. If the scratch is fairly deep, it will take lots of work. Fair warning, keep the disc wet while polishing out scratches or you'll ruin the disc.
I have used this product for a couple of years. It works ok but is a lot of work. Essentially, the device polishes the disk leaving many fine lines from center to edge (keep it lubbed). Then the fun begins. You have to further polish the disk by hand with a piece of felt. This takes a little time.