First, it's solid. Good code and no flaws. Second, it's highly customizable. With only a few lines of CSS the look-and-feel can be altered to your liking. And third, it's maintainable.
The only remarkable thing (in my opinion) is the storage of serialized vars in the database event table. I don't see why the developer has that built in, because it holds a date, which also is present in a date field of the same record.
Other than that, it's a splendid calendar solution, that will stick with me for many years to come!