DVD backup
I'm doing the same right now.
First rip in file mode using DVD decrypter (free dl)
Then load and view in DVD Shrink. Reauthoring mode. (free dl).
You will be amazed of what you see. Seems like many companies just like to load a lot of unnecessary stuff to fill up the dual layer disc and make direct copy impossible.
unnecessary stuff could be:
1) another copy of the same movie (you just need one of them).
2) another copy of disc two (i've seen that on a two disc concert DVD).
3) many sound files in more than one format (eg.,5.1 and DTS) and more than one language (you just need one).
Now if you are willing to have your backup without the menus, (only movies, with sounds, chapters are kept), you can continue with DVDshrink and finish the job. many movies will fit, some you will have to shrink abit. A shrink which leaves you with more then 85% is usually good quality, but that depends on the original DVD quality and type of movie (check out for yourself).
If you are not willing to lose your menues then you'll have to go for more advanced authoring software, but then you'll have to start paying.
With me, backing up the movies themselves with the best sound possible (1 only) is sufficient. After all this is what we buy the DVD for. Infact, many times is prefered as you don't have to work hard and waste time when you want to watch the movie (commercials, annoying long menues, etc..).
You can always use the backup to play the movie and make the original your actual backup except for those times where you realy need the menues and all the extras. This way yoo'll have the original and best source safe and make a new copy whenever you want or need.
Good luck!