GTD has turned me from one of the least organized guys in my company to one of the most.
For about a year after starting GTD my efficiency gains were modest, since I spent a lot of time trying out different systems and introducing complexity where it wasn't needed.
But after almost two years I've got my system in place and streamlined, and I feel I'm 100% more in control.
I'm now working on the perspective side - I've improved about 20% there but still needs work. I'm re-reading Making It All Work - which to me is a very inspiring look at the perspective side of GTD and how to put it to work in your every day.