So far it works as expected, but the fit on a rectangular downpipe is not optimal. The bottom is flat except for a small outlet tube, but there is nothing to keep the pipe aligned and can shift around. I used clear duct tape on the back.
The top doesn't fit very well either, with nothing to prevent in/out movement of the downspout in the diverter.
Black wasn't my preferred colour with all white downpipes.
Overall, it works, but I'll try to find a better fitting diverter if I need more.