I use it for my home folder, but you could do it for root as well. It
creates backups and then it allows you to restore from that backup. I have
never tried it to backup a whole system. It does a good job for folders.
Since Linux has no registry then theoretically backing up your root folders
and restoring them should get you back to where you were at a given point in
time since it is but a snapshot of what things looked like at a certain
point. But if you can't boot into your computer because root is messed up
then restoring it might be a problem. You could always try to do it manually
from the Live CD by launching Nautilus as root (gksu nautilus) from a
terminal.

There are two levels of Back in Time in the menu, one for user and one for
Root. This would create separate databases and separate backups with
different permissions. This is both good and bad. Good because you can
backup as root things that ordinary users don't have access to and bad
because home and root would have to be done separately. So depending on what
you want to backup there may be better and more elegant solutions for you.
This schizophrenic user and root thing is necessitated by permissions which
gives Linux its security by gives users plenty of grief. BTW, Back in Time
is just a GUI for rsync which serves as the backend.

For me Back in Time does the trick because installing and re-installing is
not scary and second nature, so I am more concerned about my data, including
all of my pictures, etc. I have a separate home partition, BTW. Sometimes I
re-install just for something to do, so I have never thought about what
others might want to use backups for.