Its a co-incidence that I was looking at Google calendar a couple of weeks ago! I have always used Outlook but you dont get that free, so I have been using Windows Live. It has a calendar but doesnt do reminders, which is a key thing you want a calendar to do. So I was doing some browsing for other options.
I'm not sure about Google calendar, as I dont use Gmail so wont get the reminders automatically. It has been very useful to have the instructions as I would not have found some of its features. Its great that you can customise it so much.
I was surprised that its used in major organisations. More mainstream than I would have expected.
I am concerned about the amount of information that Google would be gathering about me if I used it though. When I clicked on the link to go to the Calendar for the first time, it opened with my main email address, even though its not gmail.