Rated 1 out of 5 stars
I do not see the point of this add-on. If you want a small icon to get you to GCal quickly, just do this: Bookmark Google Calendar on your bookmarks toolbar. You will notice that Google Calendar provides its own favicon. Edit the bookmark, remove all text from the Name field, et voila, you've got exactly the same functionality without an extra add-on weighing down your browser.
Not quite the same
Yes, you've described how you create a bookmark, but the bookmark can only go on the bookmark toolbar. This add-on creates a button, which gives you more flexibility in where you can place it.
I freely admit that it is a very simple add-on that might not be for everyone, but I find it useful, and from the reviews, it seems a few others do too.
I can see giving a 1-star review if it didn't work as advertised, but simply because you don't want to use it? You might as well give every add-on you don't have installed a bad ranking.