Believe it or not, the shiny side of the disk is not the one you need to worry about. You can get a lot of scratches there and they're all pretty superficial: the shiny side is just flat plastic. The side with the label, however, that's the actual side that is pressed with the data. If you've got a scratch across the label side, you may as well toss that disk out.
Scratches on the bottom can cause problems for the laser, but that's usually because the laser is refracting through the scratch, not because the data is bad. A "repair kit" fills in or buffs out the scratch so it is smooth again so the laser can get to the data underneath. Rather than spend money on a repair kit, I've had 100% success recovering scratched disks (enough to rip them at least, not enough to keep using them regularly) with the following method.
1) Clean the disk thoroughly, using a mild detergent or watered down glass cleaner. Just a light wipe to get any dust off of it. It must be clean!
2) Apply a small splash of vegetable oil to a paper towel and rub it around the disk. Just circle around and around it until the disk is covered with the oil.
3) Use a dry paper towel to now wipe the oil off until it is shiny again. Be careful, as dust will attempt to stick to the disk.
The oil should now have filled in the scratches and been smoothed over enough for you to read the disk.
Do note that you should only only do this if the disk is really unreadable as it can actually cause damage to the disk as well. Clean it, oil it, rip it, and don't use it again. If it still reads and plays well, then don't worry about the scratches.