I've finished the first two chapters. Normally, I am quite allergic to the type of prose being used in books of this type -- instead of telling like it is, authors like to use wooly phrases like 're-evaluating your system of values' and other such nonsense. Allen uses down-to-earth language, that I can understand and relate to. I'm not reading this book because I want to learn how to string long words together into semi-coherent jargon, I'm reading the book because I want to do something.
Allen tells us that all of the 'stuff' we want or need to do is taking up some of our mental space. Space that keeps nagging us all the time (producing stress), and that can't be used productively for other tasks. Example: every time I walk into the spare bedroom, I see the Zalman CPU cooler lying on the table, and I think: "Yeah, I should try to install it once again."
This takes up both mental time and energy, and you never get around to doing it. You accumulate lots of 'projects' in that way: stuff you need or want to do, but for some reason you never get around to it. That is because you can't do projects: you can only perform actions (which might be related to projects).
The method Allen proposes is to record/write down everything that comes at you, to free your mind from trying to keep track of it all. The next step is to process your 'inbox' and to define 'next actions' for everything that is 'actionable'. That way, you end up with a list of concrete steps you can take to further your projects.
There's more than that, but that's the gist of it. It's a pretty simple trick, one that can be implemented with no further technology than a notebook and a pen. So far, I'm impressed.