Zentivity = Simplicity + Creativity + Productivity

The key to any system

On the subject of whether a system is valid, David Allen asks, “Does it work for you?” and “Do you feel it is out of control?”

I agree that this is indeed the key. It doesn't matter whether the system works for anyone else. Nor if it is expensive, technical or pretty. If it works for you — progresses you to the stage of getting things done, then that is the key factor.