Disappointing, too flashy and trying too hard to be extreme. It's like playing an anime.
In regards to series canon, Raiden inexplicably goes from being anti-war and hating his violent side in MGS4 to being totally okay with it and being all gung-ho about slicing people in half. In that respects the story is thematically totally out of place with the rest of the series.
The characters were all flat and I didn't really care about them. None of the bosses had any depth. The game didn't really tell me who they were, why they were with the bad guys, or why I should care.
The story is pretty lackluster too.