I am a PhD student in Surgery and have recently started implementing GTD with the "Thinking Rock" platform (no affiliation, but I am pretty happy with this multiplatform product).

I keep my PhD as a multiple cascade of little projects that range in scope from "the big Mama' (the whole thing) to individual experiments and grant applications etc. It is only early for me, but I find GTD helps me to work on those thigns you know you have to (grant applications that your boss likes but you don't really believe in...) and those that keep you motivated (wet lab work, results updates etc.)

I am also a medical professional (surgical trainee) by training, and I can see GTD will have a completely different implementation in that life. GTD has been great for me in this life as I am awful at choosing to do tasks that can be put off. In my other life, tasks are a lot more squeaky wheel and difficult to ignore. I am looking forward to figuring out the different implementation.