Years ago, before GTD, I carried a Palm Pilot for a year, then (when its
battery conked out and I lost my data) switched back to
carrying instead a pocket-sized paper calendar, which I've done for years and
continued through GTD. My paper calendars have never or almost never
been lost, misplaced or seriously damaged e.g. due to rain or from
accidentally being run through the laundry, so in my experience
they're more reliable than the electronic device. (In theory I could
have backed it up more often.) I like paper: you can write in different
sizes, on a slant etc., you don't have to spend any time turning it
on or off, you can immediately recognize a page you'd just been
looking at because of the unique handwritten markings, and stuff is always
on the part of the page you remember it being on.

I also do a lot of things on computer, and at times I've kept lists of actions
on the computer, but now I'm happy with lists of actions on paper: many of
them one action per sheet of paper (with or without other information on the
same page), and many of them brief notations in a pocket-sized notebook
I carry along with my paper calendar. (I re-use sheets of paper by erasing
if they only had a few words.)

I like the feel of lists of actions on paper. Perhaps I prefer to be telling
the computer what to do rather than vice versa.