Having just finished watching this the other day with my wife, I was amazed by how different from the norm this was, which I think is necessary to do every so often, especially when you have a long running franchise. Each incarnation may be different and the story may be different, but the way things should be approached in the storytelling overall should also be unique and different enough to keep it fresh. I mean, just because you have an established formula that was successful in the first place doesn't necessarily mean that that same formula should be applied again and again.
I rather enjoyed the darker and more adult-oriented approach to the subject matter, especially making the transformation aspect seem more lycanthropic. I also rather liked the sci-fi approach that this creature came about largely through man's attempt to play God through the use of science. I also rather loved the basis for the experiments to strengthen the body's genetic structure against infectious agents, my wife pointed out that it was almost like stem cell research without the stem cells, and also long before such a term would even be coined.
No doubt the visual, cinematic, stylistic and atmospheric approach to this particular Kamen Rider story owes a lot to David Cronenberg's version of 'The Fly', and some of the emotional imagery regarding Shin's own angst over his situation and how its affecting his relationship with his girlfriend, shares some similiarities with 'The Incredible Hulk', which I thought was really nice.
I was somewhat confused and a bit put off by the ending.
His girlfriend dies while still carrying Shin's unborn child and yet its hinted that the fetus is still alive even though the mother is dead, and the movie ends before we even see how the unborn child is saved or even brought into the world. I know this was meant to act as a pilot for a potential series and its possible that story thread would've been explored had the series been made, but it would've been nice if this thread in particular hadn't been left dangling
Other than the ending, I really enjoyed the film.