IIRC, Knoppix is a mix of the Testing and Unstable trees of Debian. 'Stable', 'Testing', and 'Unstable' are basically stages of the Debian development process and packages are grouped according to their status.

Packages in stable have undergone rigorous testing and have passed the Debian criterias for the particular release. Testing is just that -- they are undergoing testing... Unstable is the latest and the greatest... 'bleeding edge' if you will.

The 'unstable' branch of Debian doesn't necessarily mean it is... it's just that it falls under that criteria in the Debian project. If your check distrowatch.com for package comparisons with other distros, you would notice that a number of them use package that Debian classifies as 'unstable'. It doesn't necesssarily mean that it is.

There are a number of things that the distro you know as Knoppix aims for and IMO, it does the job perfectly. Unless you plan on using Debian as a server, I think you'll do well and fine with the HD install of Knoppix.