Excellent! The best.

Can be customized in many ways, but these are its salient features:
A rich choice of icons in many colours allows you to choose the right icon for a page background of any colour, be it light or dark or any other, always allowing for excellent contrast and visibility.
Two sizes for the main arrow (permanently visible, or hidden after a few seconds) make it adaptable to the free space on your pages.
In addition to the main button (scroll all the way up or down) there are buttons for scrolling only the visible area of your browser window. Very neat!
You can have a toolbar button only, thus keeping the web pages free; and the many icons that you can choose from make it usable with any theme or, again, any background colour.

IMHO the best of its kind.