Pretty nice, but I only see disadvantages in comparison to scrot. Don't see a reason to switch now.
I'm not sure what could/should I report, but slop is terribly slow. That is, I'm moving cursor and the rectangle moves after it with delay of several seconds, literally. scrot -s responds instantly.
One more minor issue is that by default slop selects currently hovered window showing the border, but if I just press Enter rectangle returned would be still 0,0,0,0 which is kinda non-intuitive.