The free version of the add-on is easy-to-use, fast and reliable, even with (very) long content. Ideal integration into YouTube's interface.
I bought the pro version with confidence since it seemed so robust, and I was not disappointed. In a minute I had seamless access to HD download, which is as easy and fast (considering the higher volume of the files) as the regular version.
I haven't found any cons or flaws yet, as far as my uses are concerned.