Someone I know wrote something like this in his blog, this week:
People have long been complaining about the one thing they are missing in the open source arena as far as email clients go is a calendar that they can share, etc. This seems to fulfill that need.
This ruffles my feathers. I admit that I am the kind of curmudgeon who actually cares about the difference between open source and free software, but here it's not even the issue. Google Calendar is not open source. It's free of monetary charge, to use as permitted, for now. I'm sure it's a reasonable web calendar, and I'll suggest it to my Win32-using friends, at least until Sunbird is ready for prime time.
To say that this helps free or open source software is wrong and harmful. It hurts open source by providing a shiny, proprietary alternative. Sunbird and other free calendar sytems need to get out there so people can use them and benefit from their openness. The extensions we'll see for Sunbird will, I think, be pretty great. I think we'll see the same independantly-developed extensions for Google Calendar that we've seen for Gmail: basically none.