I like handhelds and I like GTD. After having tried many handhelds (old Palms, Tungsten T3, Tungsten C, Ipaq 4150, Ipaq 5450), my clear favorite for GTD is the Palm platform to begin with. It fits GTD perfectly. And if you work with excel and word documents often the keyboard on Tungsten C makes it stand out from the other. - - It is big difference to put in text with the keyboard.
I can really recommend this powerful tool for GTD.