In my opinion, system backups and CVS are compliments to one another. It's not effective to employ only one. Although a lot of people will perform backups, but overlook CVS. Here's how they're different.
I'm sure you're familiar with a system backup. It provides a snapshot of every file on your machine at a given time. If something goes wrong, you CAN wipe the system clean and install from the backups. The point I'm making is that backups tend to be used to work on all the files on a system as a whole.
CVS provides a revision system. It operates on individual files. Using its check in and check out routines, you keep a history of the changes to that file and can step back for that file specifically without affecting other files on the system. This is ideal for configuration files (/etc/profile, /etc/XF86Config, bootscripts, etc. ). If you tweak a configuration and it doesn't work, then you can roll back to the previous working configuration without reinstalling the system.
Using CVS as a backup scheme for the whole system isn't a good idea though. You have to manually update every file that changes with the check in and commit commands. So, when installing new software, you can see it would be extremely tedious to find every file the install copied over to your drive in order to check it in.
If you choose to use either/both, you need to be persistent with it. If you update a lot of your system without a backup, then if something goes wrong, you'll have a lot of work to do getting from your backup back to where you were. If you make a lot of changes to a configuration file, but don't check it in, then the same thing happens here; you might have to re-read an extensive man page to remember how you managed to do something earlier.
I don't know if any of this made sense or not. I'd be willing to give it another shot later though. Right now, I'm a little groggy having rolled out of bed not too long ago... So, the above message might need a decoder ring...