http://www.distrowatch.com/dwres.php?resource=major

According to Distrowatch, Knoppix is one of 10 major distros. But to me it makes sense, it is a really really good distro - combine the power of debian with Klaus's magic and you have something special.

Also, being a CD based distro it is much more likely to be on the cover of a PC magazine than another distro like redhat because it can be safely "trialed".

It's hard to judge a distro's popularity ( definatly NOT by page hit rankings on distrowatch.com ! ) but I think Knoppix is up there in new users, i.e. out of every 100 new linux users this week x% will use Knoppix.