Easy to use but absolutely ingenious:
You just drop perfect egg timers into the water with your eggs, turn on the heat, and watch it begins to change color(from outside in). Three lines on the timer (soft, medium, hard) indicate different doneness of yolk. Then you just turn off your stove and they are ready!
Precise report egg doneness:
Although it's labeled as a "timer" it actually measures the temperature gradient as it's heated by the water. This corresponds to the rate at which the egg cooks, so you actually see an analogue of the inside of the egg.