I have used a web calendar for about 10 years and consider it an essential time management tool, working as I do in two separate locations in Cambridge. Colleagues at each library can check where I shall be working on particular days, see when I am available and inform researchers when I will be in. It is especially important during the holiday season, when I sometimes need to change my regular work days to cover for colleagues' leave. I do still carry a university pocket diary however, as I find this invaluable at home, on the move and in meetings.
I have not used Google Calendar before, but have added it to my iGoogle page as a trial. I dislike its weekly view because of the need to scroll down the page, whereas my current online diary (UL designed) enables me to view a week at a time without doing so. I shall probably refer to it most in agenda mode.