CD Repair?

I've also seen some serious commercial CD polishing machines.
Doesn't make sense for most of us but I think these services and DVD rental chains may be using them.

I've used a good quality metal polish like "Maas" to polish out some nasty scuffs and not too deep scratches from abused library cd's. It works quite well as long as things aren't too far gone. It's a bit faster than toothpaste, which does work as a good polish for all manner of things.

Sometimes work, sometimes not. I have tried that service, and the success depends on the conditions of your disc.

I would not buy a used CD that was "fixed" in this fashion. The polishing takes away the scratch but in doing so, takes away layers of plastic, thereby decreasing the thickness of the material layer between the data and the surface. This would not be a problem if the laser within the CD transport could auto-focus, but this is not the case. CD transports & players utilize fixed focus lasers. This can reproduce errors, enacting the electronic error correction -- just as the scratch does -- so no benefit has been gained, only further destruction of of the disc...

I have never tried commercial cd polishing machines. I am not sure if they really help or not. As some others have said I think it would depend on how good or poor condition the CD was already in.