It's honestly almost solid. On the first use each day, or maybe even each browser session (I haven't confirmed), it will ask you to punch in a 6-digit code to activate it. Since the 6 digit code is shown, I see no point in this - so it really could just be removed altogether. After all, I am already logged into iCloud, why do I need to "authenticate" this again? The window also seems to position itself on the top-right of the browser window, covering the input field which is mildly annoying but doesn't hinder your ability to enter the code. Once you're past this however, it's fine. It works as expected.