Like others, I found this add-on very frustrating. Firstly, I could only get it to work on a freshly created worksheet built for the purposes; adding it to an existing sheet wouldn't work. That was not so bad, as I could easily enough create a dedicated sheet that pulled the necessary data from my existing sheets.
Secondly it only seems capable of sending one reminder. Whenever I tried to set up a second reminder, the first stopped working. I even tried creating separating worksheets for each reminder, but no joy. Again, not critical - two reminders would be nice, but not essential.
Thirdly, and most critically, like others, I found that it stops working after a few weeks. I can get it to work again by opening the sheet and add-on, editing the reminder and saving it (unchanged). This works again for a few weeks at which point it fails again. This makes it worse than useless, as the whole point of a reminder is to help you not forget something.