It didn't cover content for me, but I think it's annoying anyway. Instead of using fixed positioning, which the browser will keep in place no matter where you scroll without any annoying jumps, he updates the position with Javascript just because he wants the sidebar to lineup with the top of content when you load the page and to stay near the top of the screen while you scroll.

It's stunning from how annoying it is, and lets not forget that Craigslist is also a design used by millions of people, so his assertions don't automatically mean he is a great designer, although I prefer the simple if cumbersome design of Craigslist to an annoying jumping sidebar.