I read David Allen's book last September and was very much attracted by his concept. I still work with the concept although I have realised that it has limitations and needs adaption.
One of this limitations is - putting it bluntly - that it is a manager's tool. S/he managers and others do the work. Hence the delegate and waiting for slots. The items on my to do list are items I have to do myself. And that counteracts the efficiency of GTD.
The second limitation is that educational activities do not follow the efficiency rule either. (I'm looking for a new job) More time, can do more things. Learning cannot be compressed.
Other than that I think toodledoo is a good complementary system. I think I'll use it to remember things, and to think about priorities, structure projects into tasks etc.
I wonder whether there are similar thoughts and experiences out there.