Admittedly, this is a very efficient add-on and it does exactly what it says on the tin. (thus 5 stars)
As others have already stated, it's a convenient replacement for the "Grab and Drag" add-on, albeit it is much more minimalistic than the latter. Don't get me wrong, I love minimalism in the right places, I just have to say that I would prefer the way Grab and Drag actually "drags" the page directly (1 on 1 or with an added multiplier, similar to the way Acrobat scrolling works, or on mobiles/tablets), over just virtually dragging the scrollbar. I mostly just ignore the actual scrollbar anyway.
Also, the left mouse button should be an option.
Other than those two aspects, I think this is great.