Most commercial DVD's are dual layer, with a max of around 8.5 gigs of information.
Dual layer burners are getting cheaper, but dual layer media is not, still close to $3 a disc. Single layer media is about 40 cents a disc for the good stuff, and is what most people have and use.
When copying a movie for archival, and using single layer discs, compression needs to take place....all done to the video portion of the movie. So depending on how much compression is needed, it can affect the video...but not the audio.
Some software allow you to take some portions of the disc out...such as the extras, menus, and audio tracks to create more room so less compression is needed.
I used DVDShrink, which is free, and have found it much better than DVD X-Copy.
