We suffered for years in our household with hit and miss egg timers or sheer guesswork with the result that often as not we were left with underdone or hard boiled eggs.
I was a bit sceptical about this device, but it really does deliver exactly what it promises and we get the eggs exactly how we like them every time.
We found that living in a hard water area, the timer acquired a film over the surface after a while which made it hard to read. However, this was easily removed by wetting it and then rubbing hard with a glass cloth.
One tip is to take the pan off the stove just before it reaches the chosen mark. Whilst you are fiddling around getting the toast and/or egg cups and/or tea ready the eggs continue to cook in the hot water and end up just right. Even if you take them out of the water they will be very hot and will continue to harden up until you eat them, so bear this in mind if you are not immediately going to tuck into breakfast.