Ours works great ... and our 20 month old loves it. He was waking up earlier and earlier. He would always have a hard time getting back to sleep and was just cranky during the day.
After a few days with the light, he totally gets it. We remind him that the red light means it's sleeping time. When it wakes up when the light is green, he stands up and point to the light and says "Green" and has a very happy wake up.