The CBZ format is mainly for comics. Strangely though, iRiver hasn't done a good job of supporting comics-reading. I've yet to come across a mini 6" comics. Even a small manga book is much larger than 6". You really have to squint hard or use a magnifying glass to read the words.
What iRiver could have done is to allow zooming. But this feature is completely absent. Zoom isn't available for the original and the new Cover Story (for viewing jpg, cbz, pdf).
Back to the question by Renji. Cover Story is much faster when you flip the pages, and when you navigate around, probably because iRiver has installed a better chip in the newer Story. It's also a lot more comfortable to hold in the hand. But the reflective screen is pretty horrendous. If the price is about the same, I'd say go for Cover Story.