I've been using Google Calendar since it launched (well, actually, a bit before it launched :-)) and loved it. Yeah, I've used Outlook's Calendar and it's nice and all, but without an exchange server, I can't access my calendar from home and work. Plus, Google Calendar has Quick Add which is pretty much the best feature ever.
The only issue with Google Calendar was that since there was no way to sync Outlook & Google Calendar, there was no way to sync my blackberry with Google Calendar. Boo :-(
Plaxo just released a new version though which solves this. It can now sync between Google Calendar & Outlook (and a bunch of other services which I care less about). Ahh... meetings on my blackberry now :-).