Critique on getting Things Done and David Allen

David Allen is a productivity consultant and a renowned author. His most popular work is "Getting Things Done" a book that outlines a time management methodology. He was able to gain recognition for his book in a short time and Getting Things Done manifested itself into one of the best methods for productivity in many organizations. Released in 2001, the book became the David Allen's claim to fame.
He continues to implement the methodology used in the book and has been able to garner the attention of a significant audience even till this day. For some readers, 'Getting Things Done' became so helpful that it is known to have a cult of its own.