The calendar invitation addition is really handy too. It means you can set up an invitation directly from Gmail.
This sounds simple enough but users previously had to create an invitation in Google Calendar first and add participants from Google Calendar. It wasn’t/isn’t a difficult process but it meant leaving Gmail to do that instead of doing everything from one service.
I like the UI for this extension. There are frequently times when I want to set up a meeting with a client or colleague and that’s usually when I am sending out an email about whatever it is we are going to have a meeting about. This makes the whole process a little more convenient and productive.