My wife and I share a .mac account. We’ both use the same iCal calendar to keep track of the (way too many) appointments and events we have. It’s been great, and with the color coding on calendar events, we can see what each-other is doing before we accept yet another invitation or appointment. We use the notes feature a lot too, for details about events, reoccuring phone numbers and so on.

We have one item at the top of te To-Dos list that contains special characters (from the Edit -> Special Characters menu) so we can easily copy/paste them into flavor our calendar items.