It's useful for fixing broken Debian/Ubuntu installations, as well as for using a computer that has no harddrive. I wouldn't consider installing it permanently though...
Installing it permanently has devastating effects if you do it wrong (voice of experience speaking here), however, I use it every time I need to rescue something (barring Macs).
as A rescue/Live CD nothing can beat it. as an installed system it is not good, trying to installing it will give you a headache:
his is a quote from the Knoppix website:
Hard drive install of Knoppix is not recommended for the new user and one should understand that there are problems associated with a hard drive install of Knoppix, particularly if you try to install more software or update your current software.
This is the distro I use and recommend, Why because it works right out of the box. No need to configure Everything, everything just works. It also comes as a 1 CD install that is a live CD that you can install later if you wish.