Briklin’s law of making software guiltless, as inspired and explained by Dave Winer is brilliant

Software that rewards you for doing something one percent of the time will get used (email, word processing, SimCity) and software that punishes you for doing it only 99 percent of the time will not get used (calendars, PIMs, categorizing stuff, social bookmarks).

This sums up the problems I’ve always had with any kind of calendering system, whether a paper diary or organiser or PDA or calendaring app.