The above is a good link (still valid too).

I would share your concerns about the quality of the CD. Most people burning CDs for profit are very likely burning them at high speed. This is know to cause compatability and reliability problems. I can't make any good recomendations as I burn my own. I would suggest that you stay away from the electronic bay of thieves and if you want to buy a CD at least do it through a vendor listed on the official vendor list. This link can be found by following the Get Knoppix link near the top of all forum pages. There is no assurance that these CD are made any slower or with any better materials than CDs that you get elsewhere, and if you do have problems you still need to deal with the seller and not knopper.net, but buying from an official source does help support Knoppix and it's likely that there have not been many unresolved complaints about the official vendors.