I've conducted an experiment the past few weeks. I've gone off GTD the last few weeks, retaining only a calendar of appointments.
And I have to say that I've recovered a creative, intuitive spontaneity that I has been missing from my life ever since I started GTD (in 2006). I haven't spent a minute worrying about my lists or my system. I have simply worked on what I intuitively know is most important in the moment.
Please don't get me wrong: I'm not saying that others will have the same experience. But what I've found is that my memory is more than able to keep up with my commitments---to adapt to a changing landscape of work much more quickly than a bunch of lists. In fact, GTD caused me to distrust my memory in unhealthy ways. I ended up with a bunch of mechanical lists that caused constant overwhelm and made me feel detached from life. In fact, for a personality such as mine, GTD proved the ultimate form of "alienation." I was living for my lists rather than responding creatively to the opportunities and people in my life. My humble suggestion: if you find yourself in a cocoon of lists, detached from your life, then perhaps GTD is not the right system for you.
Here I already here the chorus of voices: "Were you doing GTD right? Were you faithfully doing your weekly review?" Perhaps not. But I would humbly submit that some of the "scientific" principles presented in the book are not universal. For instance, the book suggests that if you keep tasks in your mind, "some part of you" will think that you have to be doing them all the time, resulting in constant stress. I'm not sure what research this is based on. In my own experience, I've found that my memory is more than capable of laying things aside, so that they can be recalled later. (In other words, my mind does a much better job of sorting out priorities than my "system.") In fact, the knowledge that I had a long list of ALL the commitments in my life proved more stressful than keeping them in my mind. My lists made me feel that I had to be working ALL of the time.
I would suggest that the key principle of GTD is not writing things down, but rather developing habits of mindfulness and thoughtfulness. In other words, the important thing is to process and reflect on your commitments. What I will take away from GTD is the habit of reviewing my work mentally at the beginning of each day. Sometimes this means sketching out plans on a piece of paper. But I've found (to my delight) that this is something I do much better in an impromptu fashion than through a system.