I have a music CD that I bought used and while there are no deep/major scratches on it, there are many small scratches and one smudge.  I'm kinda worried about the lifespan of the CD, so I want to know if anyone has successfully refurbished a CD before (I've only considered). I'm aware that there are online guides that suggest using toothpaste or whatever (which I personally don't trust), and there are all CD repair kits (also don't trust), but if anyone has fixed a scratched CD before, can you tell me what you did and if it worked? Or if it's even worth the time/risk?

Buy some of the CD scratch repair liquid, it actually does work pretty well.  And make sure to use a pad/cloth that is designed to use to wipe/clean CDs.

Would using such a liquid remove some of the outer coating of a CD?

No, you use it only on the reflective side, not the label side.  It fills in the scratches so the laser can properly read over a damaged portion of the disc without error.  I had a CD that was scratched pretty badly, and after I resurfaced it with the scratch remover a couple times, I couldn't even see the scratches afterward, it was like a brand new CD.

Hmmm... would that work if the CD has some of the label chipped off?

Which brand do you recommend then (there are many of those CD polishers out there...)?

The label of the CD is several times more important (though less delicate) than the "shiny" side is, as that's where the actual data is stored.  If some of that is chipped off, the data is basically gone.

I learned that the hard way after removing some CDs from one of those cheap binders that had sat for a few years in harsh elements. >_<  Parts of the labels were stuck to the plastic, and ... yeah, not pretty. 

Remember that the next time you think you're doing your disc a favor by setting it on a surface shiny side up. :)

I'm sure most of them have the same result.  Probably anything you find in a store that states it is designed to repair scratches will work the same.

Underneath the top of the label side is the reflective surface (like a foil or something) that reflects the laser so data can be interpreted.  The bottom of the disc protects the reflective surface, so even if it is scratched enough to affect the laser's ability to read the disc, the scratches can be filled in as long as the reflective surface isn't damaged.

Many video rental stores have disc repair facilities.  They have machines that are better than the ones you can buy.
As everybody else said, if the label side is damaged, there is less of a chance of saving that disc, since the data is closer to that surface.