Another thing that matters perhaps more than anything else for how "valuable" GTD is for a given individual, is his/her background and personality. Let me illustrate with a pair of extremes:
A person who saw a need for "orderly task management" early on and has practiced it for years and years, using lists of actions and projects and all the rest of it, dealing effectively with mail, phone calls and interruptions, perhaps using both his/her own methods and various "public" methods such as DIT, will not necessarily notice much difference at all when they come across GTD, likely not nearly as much as a person who has lived his or her whole life in a total chaos, with boxes of "stuff" piled up all over the office, with a mailbox at the street that you can no longer even see because it is buried in old unopened letters (this person had never thought about actually dealing with mail until he read the GTD book: he had always thought the mailbox was some decoration and found the idea absolutely brilliant that you can actually open your mail and deal with it.;-))
So who benefits more from GTD, and what specifically do they benefit?
Well, I'd say the "chaotic" person, if he/she really adopts the whole message, has made an enormous improvement. The "already orderly" person is more likely to have just "trimmed" a few aspects of his/her practices, in some cases perhaps to a significant degree. And all this is totally unrelated to their profession. There are "orderly" and "chaotic" people in all walks of life, and all kinds of mixed varieties - orderly in some ways, not so orderly in others, too orderly in some ways ...