I save a lot of old programs on DVDs and have never had one fail yet, and I have quite a few that are almost 20 years old now. I think the main thing there is to make sure you always use good quality discs and to burn at slower speeds. I only use Verbatim and never burn higher than 4x, sometimes even at 2.4x.
This site has great info regarding DVD quality.
For ebooks, documents, images and certain other files, I back those up nightly to the 2nd drive in my computer and to a 2nd PC, and that 2nd PC backs up to its 2nd drive. And in the case of ebooks, I also back those up to my netbook and to the cloud periodically for when I might be traveling and have to do a factory reset of my reader, so I won't be stuck without reading material. So I think having my full library in 6 locations has me covered. The joy of ebooks being so small in file size!