This is going to sound crazy, but you'll have to trust me a little.
I've used two different types of DVD repair gadgets and both had limited success. Maybe 2 out of 5 DVD/CD's were made to work.
Here's the "trust me" part. What I do now works extremely well for me, almost 100% success. (Remember, I already said it will sound crazy.)
What I do is sit down wearing denim pants so that the denim is stretched tightly over my thigh (this is actually important as folds in the denim will further damage a disc). So with the denim smooth and tight across my leg I put my hand flat on the disc applying pressure as evenly as possible and I rub the disc in a circle pressing down reasonably firm. I do this for about 1 minute and almost always the disc will play/copy perfectly.
Like I said, it sounds stupid but it works great for me.