Google relies heavily on Ajax (Javascript) which means those with JS disabled will be left in the cold. This might be fine for some, of course, just mentioning. Otherwise it's alot of extensive server side coding and less "cool" stuff like write-in-place event edting via Google's awesome API. But for PHP freeware solutions just google "easy calendar" and you'll see plenty of choices, including plugins you can get for Drupal, Joomla, and other popular open source CMS's.
-jim