It's a strange one this : on one hand HBO went to the expense of asking the actors who play main characters to voice different chapters (such as conleth hill, Nikolai coster-waldau, Sophie turner, Aiden gillen) and that part is done really well. It gave a short but enlightening history of the rise of the targaeryan empire up until the events of the main show.
On the other hand the animation is all over the place! Some scenes are quite pretty like a living watercolour, some are intentionally sparse but artistic (almost anime), some are like bad CGI and some literally ARE unfinished CGI!
It's well worth a viewing though to all GOT fans, but for the audio and show history rather than the animation.