I've been using it for probably about a year here at work. I keep track of all my work related business on one calendar, play stuff on another, all of my church activities on a third, my nephew's soccer games on a fourth, and the St. Louis Blues schedule on separate home and road game calendars. (Separated because I'm a season ticket holder.) US Holidays and Birthdays also have their own calendars.
I'm waiting, patiently, for the iPhone to get colored calendars so I can see stuff easier.
I'm waiting, patiently, for the iPhone to get colored calendars so I can see stuff easier.