Scrollbars are virtually non-existent on Macs these days. Plus this works well on touch-based devices too. Overall, I like the concept and find it useful as well.

Edit: I'm referring to the reading progress bar concept, not to the loading status referenced in the original post.

Fair enough, and good point on the touch-based devices, I can see how it would be useful for those. I don't use a Mac, so can't comment but same applies.

Scrollbars aren't but they still show on a scroll event (unless I'm mistaken - I'm only using 10.8).

A lot of comments or post-content links/ads disrupts the usefulness of the OS scroll-bar.