First of all, there's...a painful averageness about this anime. I wouldn't call it particularly bad, but I wouldn't call it good, either. I wouldn't stop someone from watching it but, at the same time, I wouldn't encourage someone to watch it either.
There's definitely some level of potential underneath the surface of this anime, but I feel the same way about it as I did with B: The Beginning. It should never have been 12 episodes, and suffers greatly from being so short, as it fails to cover certain things or elaborate on its plot enough in the time that was given.
There were a couple episodes where they focused on different characters than the main cast. While broadening the cast would be a good thing, and while I'm sure that those characters are essential to the source material, they serve no purpose. One episode in particular introduces 3 new characters who are clearly enemies - or, potentially, enemies - that never get seen again.
The plot itself has some interesting stuff about it. There are tons of demonic weapons throughout the earth that basically possess their users and turn them into evil beings whose entire goal is to kill. Some people aren't immediately taken over by those weapons and can harness their powers to fight, but will eventually turn into those demons themselves. There is an organization whose goal is to gather these weapons and keep them out of humanity's hand. They work with these not-yet-demon humans to find and store them.
The main character sucks balls, and the series is barely kept afloat by its somewhat interesting side cast. Gai, the titular MC, was born holding one of these demonic swords after his mother, while pregnant with him, was possessed by one and she decided to kill herself. He's cold to everyone, treats even the people who care for him like garbage, doesn't benefit from MC power ups, and just...sucks. It's hard to be invested in him as a character, or his story, given that he has pretty much no development in 12 episodes.
Honestly, I think with a second season the show could be pretty alright, maybe even good depending on where it goes. It certainly ended well enough for me to want to read the source material, but...there's no translations available, so scratch that, I guess.