I am using Knoppix 5.1.1 installed on HD for over a week or so and all i can say as for a desktop user, thumbs up. I had some problems with the CD version so i vote for the DVD version. It meets all my "loser-user" needs as dvd/divx, mp3, some games, emulators, OpenGL, office work, internet (kopete is so cute btw, KDE ftw) etc etc. I downloaded some games like: Cube 1 & 2, Open Quake 3, Alien Arena. I mannaged to sucessefuly update WINE to its latest version and now bunch of a must apps (for me at least) work, like WhereIsIt and WoW . Haven't got any stability problems so far and apt-get works as well as it should.
Some other users may have different experience with stability I guess, but that may depend of your hardware. Mine is ATA system with Athlon 2000+, system just about a year and half old. I won't advice installing the CD version since it does not include the kernel source needed for the nVIDIA driver and you will probably need to recompile the kernel and for a newb its very confusing.