There are a number of very good group calendaring products.
If Exchange/Outlook is not your cup of tea an dfor many people it is not.
I would consider Hyperoffice and Zimbra to be the best. They each stuff lots of services that other system cannot do including single sign-on and getting updates from various calendar sources. Zimbra is the best with dial-up or lower speed connections. They play well in both MS and Linux environments. You can also limit who can see your calendar, to a select group or everyone in the world.
If you are in a Linux shop I love http://www.opengroupware.org/, it is the complete package and run great along side redhat or ubuntu. Nice Palm and Windows Mobile access.
If you work in groups try http://www.wiggio.com, I know people who swear by it. Me, not so much.
If you want free both Yahoo and Google offer calendaring functions. I have used them both and they both work well and essentially have the same fuctions.
The most underated group calendar is localendar.com. They have zero money to advertise but can compete with the other guys as far as services are combined. My worry is will they be around in 6 months.
WallyDallas what is in gcal that is not in Outlook/exchange calendaring function. Everything you mentioned I am doing now with outlook/exchange (and no third party apps) plus people do not need to know another logon. I use a WM 6 phone and have access to five distinct calendars. I also have acces via phone or internet with one logon.
Gcal is a very good caledndar program but IMO, is not a group calendaring program but an individual calendar program. Just try doing an automated update of multiple individual calendars in which you want select items (not all) to be updated?
I am unclear how you rate virus getting with calendaring software but if you go to Sans www.sans.org you will see on a per-email / per-per person gmail get 17% more viruses than Outlook 2003 or 2007. There are a lot of people who prefer not to use MS, I get that but there is no need to trash a product based on old information, rumours, or hearsay.