Plot/Story
The main focus of the story is Nana and Kaoru's relationship. Which consists of their S&M relationship and their feelings towards eachother. Most of the chapters deal with whats refered to as "Nana's breathers", which are acts of S&M. The chapters which aren't about their "breathers" deal with their feelings towards eachother and their relationship as childhood friends and S&M partners. In my opinion this manga definitely fufills its intended purpose. Its very erotic without being too explicit and it comes off surprisingly deep for this kind of shounen/seinen ecchi manga. More so than others of its kind.

Characters
Nana and Kaoru are opposites. Nana is the smart, responsible, bossy type who is thought to be "Perfect". Kaoru on the other hand is perverted, less intelligent than most, and weak the kind of person most people would think of as a "Loser".
On the other hand, Nana can be fragile and she likes letting go of the reins and being dominated by Kaoru. Kaoru too, although hes weak and a "loser" he can be courageous and strong willed when it comes to Nana.
Drawing Style
This mangaka is one of my favorites so I might be a little biased but I love the art work. Most of the time in this genre of manga the art is somewhat simplistic, but Nana to Kaoru is detailed and gorgeous.
Enjoyment
I laughed when it was supposed to be funny and I got a stiffy when it was supposed to be erotic. I didn't get bored reading it and look forward to reading more.
Overall
This manga definitely delivers. Its got a good premise, its sufficiently erotic and arousing without being explicit, and its funny. With other ecchi mangas theres always a chance of it getting old quick and losing interest, but Nana to Kaoru can keep you hooked and wanting more.