I find the actual, physical, tickler file invaluable. Sometimes I'm not very good at installing habits, but this one is INGRAINED. When I get to work, I boot up my computer, process my email in box (another habit I've got down), and then I would feel TOTALLY weird if I didn't swivel my chair around, open up the file drawer, and pull out that day's tickler file contents!

Often times it just contains something I've put in for inspiration or something funny, but I deal with a lot of travel arrangements, conference registrations, etc., and being able to just plunk something in a folder and forget it until the day I need it is one of the simplest and most elegant parts of my GTD system!