My 2.5 year old was waking up anytime between 5-6am and calling for us right away. Once we started using this clock (just the colour changing function, not the audio alarm), he immediately got with the program. He always waits for the clock to turn green before he calls us and often now spends time playing quietly in his room even after the clock has turned green. We now often don't hear from him until 7am or even later, although we set the clock for 6:30am every morning. I think the best thing about the clock is that it gives our son back some control. Before, he would have no idea what time it was when he woke up, and so would just call. Now he knows whether or not it's too early, and can make his own choices about what to do.
The clock has plenty of great features we don't use, like the nap timer. The one extra feature that has come in handy is the auto-off, which decides how long the green light will stay on. It defaulted to 30 minutes but we have reprogrammed it to stay on for 2 hours, which lets our son sleep as long as he wants and still wake to the green light.
I suppose if I wanted to list a con or two, it would be that the back is fairly easy to open, so we have to keep it out of reach of our son. But overall this clock is perfect for anyone with a toddler who wakes too early. It is also quite expensive for a little plastic gadget. However, I would gladly pay it again for the peace it has brought to our mornings.