Well, as the pussy cat said to the owl, it all depends on how much data you've got, and how often you might need to access it.
While I certainly wouldn't dismiss backing up onto CD-R or (to a lesser extent) CD-RW, here's a somewhat different view.
I do a lot of computer literacy testing and have to keep all the files associated with each test for 42 months. All the files associated with each test are kept in an individual folder, which can vary in size from about 50Kb to somewhat more than 1Mb. Candidates need to complete seven tests to achieve certification. This creates a problem because candidates may do the tests over a period of three years - so keeping them organised chronologically is not a good solution because accessing a single candidate's cumulative results could mean a lot of CD shuffling. And worse than that, a query for a single candidates results doesn't give one a clue of when he/her might have done their tests. This is a significant problem because I currently have around 10 000 folders (about 18GB of data).
When it comes to back-ups, a truism is that the more difficult/inconvenient and slow the process, the less likely it is to be done on a regular basis. And writing back-ups to CD Rom is neither particularly convenient, nor particularly fast, and is labour-intensive (unless you've got the luxury of a magazine-fed burner that plops a fresh CD into the carrier when so requested - a device you won't easily find down at your friendly computer store). Also you need to label and store your CD's somehow - not a trivial task. And I have some doubts (and somewhat irritating personal experience) of the data on CD's not always being available when actually required.
So, I rely on two forms of backup.
Firstly, I have two hard disks installed (40Gb), and the entire operational disk is mirrored onto the second disk on a regular basis. Which (hopefully) takes care of the possibility of the operational disk croaking and/or having an unconstrained out-of-case experience. You just have to resist the temptation to use the second disk for ad hoc storage.
However, this would not help if some local villain took a liking to my system and removed the entire system.
So, I have a 3rd disk in an external IDE enclosure which plugs into the USB port. Cost for the enclosure and the 40Gb drive (which one buys separately) was less than 100 pounds - cheap for the peace of mind that it buys me. This drive (it's about the size of a paperback book) gets locked away well away from the system.
The initial copy (currently 27Gb) is a kingsize pain, but the incremental back-ups aren't too bad. And at least you don't need to shuffle CD's, nor do you need to exercise grey matter in deciding what you do or don't need to back up - just back up everything!
Such a system may or may not suit your needs. But it is an alternative to paper and/or CD's.
For what it's worth ....