This is my first in this genre, and I gotta say it's a completely different world from all the manga/H I've read so far. No maiden desu-desu, no harem, no fight scenes, no handsome male protagonist, and no complicated philosophies - just a simple story about two distanced childhood friends, who by chance, are once again reunited through their secret adventure together.
I love how NtK can be so erotic, laying bare the humanly nature of lust, lewdness and desires. In many ways, it is far more revealing than most H-manga out there, which I believe merely serves as a quick physical release...
More importantly, the beauty of NtK is that it hardly ever make use of (if at all) explicit sexual acts. The plot, dialogues and character developments blend extremely well, and the characters are portrayed with with real-life personalities. Of course, the gorgeous artwork just made something perfect even better.
I noticed many comments about Kaoru's fugly looks. I agree, it was a quite a big turn off in the beginning. However, this became increasingly irrelevant as we get to better know his personality. Yes, he is a BDSM fetish creep whom you'd probably not want to meet in real life (but hey, who doesn't have some kind of fantasies?), but he would do everything he can to satisfy Nana's needs (as well as his own...). Moreover, his bad looks is essential to the plot in order to fulfill his role as an unworthy match to Nana to the outsiders' eyes, as well as to keep his own self-esteem low throughout.
Up till ch 51 and arashi 9, we can see how the limit in their BDSM experience is getting pushed further and further. I am really looking forward to seeing how the story unfolds when (if ever) they get beyond the point of acknowledging/confessing their love and/or really doing it.
Lastly, this series is probably be directed at a more mature audience, given the BDSM the theme. Nonetheless, it did opened my eyes on this theme of softcore BDSM and in the psychology behind the play... if it were ever close to reality.