It seems simple to set up and as things inevitably get in the way it will shift around your appointments to find the time for you to meet that goal. It seems like a cool feature especially for me as I often find things to fill my day so I can avoid going to the gym! Goals will soon reschedule around one of my "reasons I can't go to the gym" and say "hey you could go at xxxxx instead". Great :-( (I hate the gym!).
Not sure if this will be a permanent Google Calendars feature or if its just one of Google's test projects but its an interesting thing anyway.
To find out more about Google Goals click here.