I have been a heavy iCal user ever since I first purchased the PowerBook G4 and I still open it every day. However, after Merlin Mann mentioned how much he liked the Google Calendar in one of the recent MacBreak Weekly podcasts, I decided to give the online service a try.
Here is a few tips I found in the first few weeks:
1. Use Keyboard shortcuts
You will be surprised to find that navigating and editing Google Calendar entries in the web browser can be faster than using a native desktop application. For example, press "d/w/m" to switch between Day/Week/Month view, "n/p" to navigate to next or previous Day/Week/Month, "t" to jump to Today and "c" to create a new event.
For more the full list of see Google Calendar Keyboard Shortcuts.
2. The Fastest Way to Add an Event
Simply clicking anywhere in the calendar grid will display a prompt to create a new event:

3. Sharing is easy
Sharing is one of my biggest pet peeves about iCal. As far as I can tell it is not possible. Even if you have a family .Mac account your family members cannot add or change anything in your calendar.
Google Calendar makes it easy — simply enter the e-mail addresses of the people you want to share your calendar with and also specify the permissions:

If you do not want to give full access to all your events then simply create and share another separate calendar with your family or colleagues.
4. Ask Google Calendar to send you a daily agenda every morning
It is like a having a personal secretary deliver the list of all meetings and appointments for the day.

5. Use private .ics URL Link to automatically see all events in iCal.

You can add this link to iCal using Calendar > Subscribe command:

The added calendar will update itself automatically but will be read-only — you cannot add or change the events using iCal. If you need a bi-directional synchronization, take a look at the Spanning Sync software by Spanning Sync, Inc.
Conclusion
Google Calendar provides a great addition to the desktop calendar program on Mac. It can even replace iCal completely but why choose when you can have the best of both worlds!