Product review (3 stars): Simple logic beats widgets.
We don't use this, because if you can remember to push a button, you can remember to fill the detergent cup.

If you always fill the detergent cup before you put a dirty dish in the dishwasher, you automatically have an indicator as to the state of the dishwasher. No need for a widget.  And you had to fill the detergent cup anyway!

Our current dishwasher has a "clean" LED that appears to simply work off of that logic -- if the dishwasher has caused the detergent cup to open, at the end of the cycle the "clean" LED comes on.  Simple. The only advantage of the LED over observing the detergent cup is that you don't have to open the dishwasher to see.