The difference between these custom scrollbars is that (in modern browsers) they are still scrollbars that work exactly like other scrollbars and only look different.
Except they don't, try using the arrow keys to navigate in the scrollable DIV. Not working for me (Firefox latest stable). It also scrolls ridiculously slowly using the mouse wheel. So we're actually going backwards in functionality.
Fuck the design if it's unusable.
Doesn't behave like a native widget for me (firefox 8.0).