Reader Mode is really best-suited for longform article consumption, IMO. So reading an individual article in CNN works well in Reader Mode, but you wouldn't want to necessarily browsing the whole site that way. I'm not sure it would even work.

(EDIT: Nope. Front page doesn't even show the option. I believe Firefox expects some kind of meta tag to let it know that the page in question has a clearly-defined article/content body.)