Well, the beginning is awful and may put off many people (including me before I came back to it a couple years later) because of the romance which is unfortunately the highlighted focus of the anime. I myself am not truly devoid of making the Twilight connection in my head because the first 3 episodes had established so and ended the 12-episode cour with that in my mind. But I'm getting ahead of myself, let's start from the beginning.
The start of Anzai and Tsukasa's relationship feels unnatural, forced, and rapey, even. The girl had trouble with dating and couldn't reciprocate the feelings her classmate had for her only to later randomly fall in love with a dude who kissed her all of a sudden. The dynamic between them had to start somehow, but it did on a bloody fan-fiction level of writing. A formerly closed-off girl falls in love with an emo vampire with bags under his eyes who "saves" her - how can you not compare it to Twilight after seeing this mess?
Just when I thought I would end up laughing watching the remaining episodes, the fourth and fifth episodes happened. The introduction of Lee (which I find to be the best character in the show alongside later introduced Ishimaru) spiced things up, as he expanded on the whole vampire thing. Shortly after, we were introduced to a certain fraction, which really started the world-building process. It took me a couple more episodes to realise that Devils Line wasn't strictly about romance anymore and instead focused on the grand moral issue of humans and vampires coexisting, all the while slowly but surely connecting the dots and characters which purpose was to build upon the narrative.
Now, I must say, the characters aren't the best, and they seem to be lacking proper treatment, but it's not like they are complete cardboards either. What I like about the show is that it doesn't throw away characters, and many of the supposedly episodic ones end up returning at some point. They also have their own motives and don't exist only to serve a single purpose.
There are some pretty exciting twists in the story, and I like how everything smoothly transitions. Unfortunately, the romance thing hadn't died off. It's still a fairly promiment matter that I've purposely omitted to show the good sides of the anime, which I cannot understand how everyone fails to see. And even despite the romance not dying off, it's now handled way better. The couple tries to understand each other, and they show some kind of maturity (Anzai's a little bitch, though, lmao). The other thing that showed potential is that their love story serves a purpose to Anzai trying to fight off his inner demons, and in that sense, it makes sense. I won't, however, forget how cringy some of the scenes could be, and I just really wish there was no romance at all.
I also had slight problems with the animation, as the characters didn't look great and had these weird white outlines. The background was pretty plain most of the time, and the action sequences were meh, but it wasn't anything jarring, and I doubt it will bother you that much.
The adaptation isn't full, so the ending left much to be desired. They went in an interesting direction, but the entangled cheap love-dovey aspect was, sadly, sigh-inducing. I can't help but get frustrated at how unnecessary this romance thing ended up being most of the time.
But even despite the noticeable flaws which a lot of people have also pointed out, I realised halfway through that I was simply having fun. I became interested in the plot outside of the silly romance, and it was my intention writing this review to shed some positive light at the story and the world-building.
