I also like this kind of stories, so I've watched it. Well, it's kind of repetitive, so it might get boring, but I still really enjoyed it. However, I don't like how they changed plot between seasons. In my opinion it was consistent in every season, but when you put it all together, then it isn't. Of course they explained it, but it wasn't convincing for me... Oh, and I don't like the 3rd season. It looks the best, but plot is really awful for me. I've also heard many bad things about this part and I agree with most of them.
But still, I really enjoyed it. Especially the first 2 seasons. The atmosphere is great, the music also. And the graphic... It's marvelous for me
However, I haven't read manga. I've seen only few random pages. And from what I've seen it's in totally different style. I mean that characters looked really different. I don't know about the plot...