Strengths: How to manage the never-ending flow of taskings and redirected taskings for those who work in unorganized cultures. Fairly simple. Can be implemented without fancy tools.
Weaknesses: Still essentially prioritizing emergencies. You may believe that if your in-box is empty you were effective today.

To balance the weaknesses, read Steven Covey's "7-Habits of Highly Effective People", and the book on Habit 3, "First Things First". The older, pre-Franklin-Covey merger book is better than the current offering.

The weakness of the Allen book is that it does not force you to think about your roles and goals, leaving you in the trap of the urgent, being unconsciously unbalanced and never asking the question: How can I prevent these issues, how can I keep the main thing the main thing, and what about the long-term?

The Covey offerings are a little weak (only a little) in handling the myriad and changing tasks that disorganized managers and organizations throw at you. Covey assumes you have a fair about of autonomy in your work life, that you are responsible for results, not for performing tasks.

Bottom line: Read (in this order): "7-Habits", "First Things First", and then Allen. In a hurry? Read First Things First, then the others in the order indicated.