Nice ... But ... Rated 4 out of 5 stars

About 1080p option, could you use the cache of FF after I played the video at 1080p ? thanks for this addon =)

(Edit) ... When I merge audio and video using ffmpeg at 1080p, the file gets out of sync, but works fine on others players. I think It is due my codec issue. Thanks for nice explanation and answer me. Continue the great job =)

This review is for a previous version of the add-on (1.7.9). 

You can't download 480p and 1080p anymore because YouTube's code no longer includes the links to those files. The YouTube player uses separate streams for audio and video, which could be downloaded, but you would have to merge them using a software like ffmpeg. YouTube streams are no longer cached: YouTube uses adaptive streaming and slices video files.