I back up all my user files (including my PDA sync files) periodically to optical media (currently CDROM), which I then store in a fire-proof file cabinet. However, I don't have an automated backup system and I don't backup as often as I should. I have a spare hard drive (somewhere) that I intend to use for more regular intermediate backups, but it's been packed away since our move last november and I haven't found it yet....
That being said, a few years ago I needed to retrieve some files from a 10 year old Mac laptop. I checked, and the machine still booted and the files were there, but I didn't have time to extract the files at that point. Unfortunately, the machine died on the second boot.
Feeling fatalistic about my chances, I dug through my box of 3.5" floppies. Not only did I find full backups of the files I wanted, but I had apparently organized them better on the floppy than on the original system! Fortunately, I still had systems available with floppy drives compatable with that format. At a later date, looking for files from an even earlier hardware iteration, I found some even older 3.5" floppies in single sided low density Mac format. As it happens, I still had a relatively old system available that could read those disks, but only one and it was quite a while before I found it. Then I had the challenge of getting the files off of that system and onto a newer system... with no ethernet on the old system. I think I eventually found someone with an old Zip drive that would work on both the old and new systems. If that hadn't worked, I was planning to try a null modem and kermit.
Then there was the time I decided to try to retrieve some old Apple ][gs files... in an old graphics format... I remain amazed at how much conversion software there is out there (and at how we are able to find such things these days, with the wonders of a searchable web).
Moral of the story: keep forwarding/translating your files to your new system as you move forward. It saves a lot of trouble. And stick to simple, non-proprietary formats (like HTML) when possible. If you have to, you can always write a script later to convert such files to whatever the hot file format flavor of the day is.
Edit/addition: I
almost lost the whole contents of a web site in that November move, as well-- because we changed cable vendors and I had forgotten that the web site in question was hosted in the "free" space provided by the cable vendor, as most of my web stuff is now hosted elsewhere. Fortunately I had local backups....