What's so great about Knoppix?

I'm just wondering, because I keep hearing about it and I've read a little bit about it. From what I understand it's just an OS that runs off of a CD. A version of GNU/Linux, I guess.

So what makes it worthwhile? I mean, can you write data with it? Because it's on a CD...

Anyways, I'm a total newb when it comes to Linux and this Knoppix thing has been bugging me for a while.

WHAT MAKES IT SPECIAL!?

Giving more information than ``it doesn't work'' would greatly improve your chances of being helped.