Some of the courses do come on CD. I have Natural Brilliance in both cd & tape.
Quite honestly, I've not used the cds because I don't have a personal cd player (hand-held).
I am actually going to rip my NB cds into mp3's and put on my mp3 player. Same for my Tony Robbins cds. That's the convenient way to carry them, I can carry 6gb of data at a time all in the same size as a cassette player. 
Tracey~!