There is one thing that it little known:

People believe that the "shiny" side is most critical - if you watch people placing CDs onto a surface, they always turn it upside down to protect the "shiny" side from scratches... well, WRONG!

There is a "mirror" layer inside the CD and that layer sits very close to the "printed" side, while the "shiny" side contains a thick layer of transparent plastic.
If you get scratches on the "shiny" side, then you have a good chance to polish it away, because you have a certain amount of transparent plastic to work on.
But if the "printed" side gets scratched (or otherwise affected) you will destroy the "mirror" layer much quicker and if this happens you can throw it away - no chance of recovery.

There is a huge controvery about the use of CD label stickers:

1. Some say that using labels will protect the "printed" side from manual damage, see above.
2. Some say that using labels will destroy the "printed" side much quicker due to chemical processes between the glue and the CD surface.

As far as I see, no final result yet...