I don't see much of a problem with the story. (I'll only touch upon surface level stuff.) Taira's a human girl who finds herself falling in love with a devil (or vampire) after discovering that one of her friends is a devil as well. This one incident has already drawn her into the world of devils. When she's rescued by Anzai, she becomes involved with the police (and her relationship with Anzai begins). Because of this, the story doesn't seem to be all over the place or anything like that. You can pretty much expect the future events in the series. My only issue is the ending. It was good but I would've been nice to see what happened to everyone else as well.
Other than the times when the characters do something stupid, they're relatively good. Taira and Yuuki are likable characters. Despite this, there isn't much development. The male and female lead characters seem to have little to no change...separately. I feel like their relationship changed more than they did. (It was still nice to see them get closer to each other though.) Everyone else either wasn't very important or they got no development. The only exception to this would be Nanako.
The sound is almost always good. I really like the opening and ending songs. "Eclipse" had good visuals but the ending song...the visuals look so awkward. The dramatic music was good as well. I wish the voices could've sounded better though. During some scenes, usually the ones when he's transformed into a devil, Anzai just makes some pretty weird sounds. They sound like gurgling...
Anyway, the art is usually good. The bags under the eyes could look better. Also, I think they could've did better with the scenes where the devils are transformed. The animation looks weird sometimes, I admit that. For some people, it may be distracting.
Story: 9
Art: 8
Sound: 8
Character: 8
Enjoyment: 9 (Things such as certain sounds and visuals were a bit off putting.)
Overall, I don't think Devil's Line is as bad as it may seem. Even stories with a relatively unoriginal premise can be good in some ways. I give this anime an 8.
