As a matter of fact, I just tried Knoppix yesterday. It loaded flawlessly from CD. I also tried Ubuntu the day before. Both OSes run at the about the same speed, however, Knoppix bundles a lot more software.
Are you planning to install it onto the HD? then i highly recommend ubuntu over knoppix. You can just apt-get the software you need afterwards and i find it much better than knoppix when it is gonna be installed. Knoppix is only better in my opinion if you gonna run it from the cd.
I recommend getting the newer ubuntu or kubuntu edgy 6.0 release though