Amazing. Absolutely amazing. Actually, amazing isn't even the word - more like fantastic.
The characters are realistic and distinctly original and non-cliche. Teru isn't the type of heroine who is kind to everyone and cries in every chapter, who has no flaws but clumsiness and trusting everyone/the wrong people. She doesn't make every mistake humanly possible, and she isn't stupid, or even extremely smart. She's just - normal. She makes mistakes, she insults people, she lies on occasion, and is, while far from perfect, in general a good person.
Also, in many other manga, the male lead is usually the most popular boy in school who seems to have no flaws (and really shouldn't notice the heroine at all, but does for some strange reason). That is most definitely not Kurosaki. He's violent, he's got a short temper, he's perverted, he smokes, he's lazy - again, inwardly a very good person, but he's HUMAN, which is so refreshing.
...And the rest of the characters are great as well - and all the dialog and character interactions are great. And you can really see how all of the cast - especially our two main characters - grow and develop as you go along. It's fantastic.
AND THE PLOT. It actually has a plot. An excellent plot, which builds and grows and when you re-read the manga, you'll go, "...Wow! I can totally see why this happened now!" But even when you first read a chapter which later leads into something bigger, you're not lost, or confused. You're right there with the characters, and likely you won't even realize what just happened was important until later in the story when it all clicks together. And nothing happens as you think it will - there are so many unexpected twists in this manga that eventually you give up trying to predict what will happen next and just go along for the ride.
Also, even between all the drama, there's something to make you laugh. The humor isn't usually (/is almost never) in an inappropriate place, though you may not see it coming, and there's something worth giggling/quoting to your friends in every chapter.
SUMMARY (for those of you who didn't want to read all that):
Characters: 9/10 (well-developed, original)
Plot: 9/10 (interesting, twisty)
Art: 8/10 (very simple, but nice)
[color=red]Edit:[/color] Well...frankly, I'm not nearly as into this manga as I was at its beginning.
I feel like romance and humor has sort of taken a backseat to lots of back-stories and a confusing and detail-oriented plot. The humor hasn't really changed or matured much over the volumes, and I can only read "I hope you go bald, Kurosaki!" so many times before it stops being funny. To put it simply, with all the action and mystery and drama and psychological-ness, this doesn't feel like a shoujo anymore, but rather a mislabeled shounen manga. And shounens are nice enough, some of them, but that's really not what I was expecting when I started reading this - I like the fluffiness and romance that the shoujo genre is known for.
So the rating has gone down for me - currently, it's at 6/10. But, I haven't yet grown irritated enough to drop this, so maybe it will get better as time goes on.