The heading showing up dynamically in the margin as I scroll I found to be quite distracting while reading. Text fading in and out while you scroll and read only serves to compete for your attention.
I actually do like the marginalia, like in Tufte's books, side boxes as a sort of commentary or summary of the main text. I'd prefer them to always be there. Lets face it, on a widescreen monitor of modest resolution, the page looks thin otherwise.
