Note: Regardless of the score I give it, I rate this manga somewhere between a 7 and 8.

Many people that are familiar with Hiroya Oku will have come across him through Gantz. With the level of fame that Gantz received and it's fairly long lasting reputation, one can't help but compare his other works to it. The scale of the manga was impressive and, though I know there are many contrasting opinions on the work, I thoroughly enjoyed it.
I mention Gantz here, as I feel that Inuyashiki bears many similarities to it.

Short summary of start:

The MC-Inuyashiki Ichirou-is a middle aged man, though, due to an illness, appearances would lead you to think otherwise. He his unhappy with his life, which is uneventful except for the constant arguing of his family. Following discovering he has cancer and only a short time to live, alongside his inability to tell his family who he feels would not care, he goes out alone to a park to sit and cry. While there, a spaceship crashes ontop of himself and one other person, killing them both. The aliens on board, in an attempt to fix their mistake, rebuild the two as machines that simply appear human before leaving again. The two retain their personalities, but are granted abilities as a result of their new bodies. Inuyashiki, who has always felt helpless, decides to use his newfound powers for good, treating the ill and saving lives while the other, a highschool boy, becomes conceited, believing himself above others and, simply because he can, decides to use his abilities to kill others and make himself feel "alive".

Thoughts etc.:

I think the idea behind the story is a very interesting one: what do people do when suddenly granted huge amounts of power.
Our two MCs are polar opposites and therefore destined to clash due to their differing opinions. The first part of the story is very well done, there are some great chapters where we see the two figuring out how to use their powers (both unaware of the others' existence) and how they change as a result of them. The tension is handled very well and when they finally meet there seems like great promise for the story.

It just feels like it accelerates too fast.

One thing I think you can never fault with Oku's work, is the art. The attention to detail is astonishing at times, faces are very expressive, characters are both threatening and sensitive where they need to be and, though the backgrounds can often be obviously photographed to speed up drawing times, it creates a great sense of realism that brings the story new life. The mechanisms of the bodies I think are the most impressive feature of this manga's artwork, the creativity and detail makes each page very interesting to read. The character designs in Inuyashiki are much more realistically drawn than the average manga; this can either work to your taste or not. It's something I've seen criticism over, but personally I think it aids in the storytelling.

There is an odd sense of the ending of this manga being both very good, but somehow rushed at the same time. I think the way it ends itself, was well thought out, though certain details of it seem like bizarre additions. I won't go into too much detail so as to avoid spoilers, but when threatened with a world ending catastrophe, both Inuyashiki and the boy end up using their powers together to help divert it consequently killing both of them. This comes only a few chapters after they had been fighting a battle that threatened the whole of Japan, a battle that obviously displayed the antagonist's lack of concern for the populace. This is what makes it, to me feel both rushed and conclusive. If there had been some event that brought some more middle ground between the two MC's the ending would have flowed more smoothly.

To summarise I do think it is worth a read. There are some definitely great moments that will stick in my memory and I did like it overall. I just feel that it didn't reach its full potential.