Well, it ended as well as expected. And not just because I heard that it has an inconclusive ending. I'm actually fine with that ending and when it comes down to it, it was either that or Rito picking Haruna. I can't really see any other conclusion.
Which brings me to the two biggest problems of Darkness: The harem plan and Momo. The harem plan, which was doomed to fail from the start, since it's just not something that fits to Rito, should have been an arc of barely even ten chapters. Instead, it encompassed the entirety of Darkness, not always overt, but at least in the background, and it's its beginning and the end. This also causes Momo to become more and more unsympathetic as it went on, as it became clearer that the whole plan was just rooted in her being utterly selfish. But since the harem plan went over the entire series, her issues went mostly unaddressed until the end, where she doesn't even get better and just goes on like before.
So yeah, thanks to Darkness, Momo has turned from a likeable character to an utterly awful bitch. And the excessive "screentime" she gets over other, better characters, severely hurt Darkness. It's largely due to that that it's worse than the first series. Mea and Nemesis were pretty good, though.