I should probably say right up front that I wrote a dissertation on the Mabinogion last year (although on the Fourth Branch, rather than the Second), and therefore have Strong Opinions on it. That should probably be a standard disclaimer on most medieval-based stuff I review, to be honest. Branwen has never been my favourite Branch - I like Pwyll for having vaguely non-horrible characters, and Math for having a set of entirely amoral bastards for protagonists, but with Branwen, the poor girl just spends the entire story being kicked around by her brother and her husband and having terrible things happen to everyone she loves until she dies of a broken heart, and who can blame her?
So actually, I like this modern retelling rather better than the original. At least Branwen finally gets to have her own voice for once! And Efnysien/Evan's actions have some explanation rather than being a pure act of disproportionate spite. The thing I didn't like was that as soon as I figured out who everyone was meant to be, it was entirely obvious what was going to happen - even down to the 'twists' that linked the Matthew/Branwen story to the framing story. I was enjoying the framing story much more than the Branwen one, to be honest, so I'm sad we didn't get told what finally happened between Rhian and her brothers.
So yes, it was kind of predictable, but that's pretty much expected with modern retellings. What really got me was when Ben gave Matthew a copy of the Mabinogion and was like 'You should probably read the Second Branch before you marry my, Bendigeidfran's, sister, Branwen. Just a thought.' And Matthew didn't! Not until everything had gone to hell in a handcart, and while it may be believable that in a rushed wedding he wouldn't have had time, and then in a miserable marriage he wouldn't have wanted to, it was so immensely frustrating to know that this guy had a roadmap of exactly how everything was going to go down sitting next to him the whole time and he never once looked at it! Rage.
I just don't know, with this book. It didn't sweep me away with poetic beauty, which is what I'm usually looking for with mythologish books - there was too much grim reality for that, which is fine in its way, but not what I was looking for, and then on top of that, it was overly predictable, but there were some lovely bits, like basically any time Ben talked about ravens, so oeverall... Eh.