This manga simply should not work.
The male lead is designed to be unattractive both Physically and in his personality.
The female lead is his childhood friend. She is meant to be his romantic interest, but is designed to be utterly unapproachable for him.
The central theme of the manga is a serious take on S&M, outside of a Hentai, and is the only thing that brings the characters together. While an Ecchi manga, it deals with the subject with next to no nudity; let alone sex.
It is episodic, most storylines depicting the events around one of their 'breathers', which is their name for their S&M sessions.
Amazume takes all of that and makes it work brilliantly. The art is good, typical of the author, but what makes it work are the characters. Both of the leads are fleshed out realistically, both are shown to be flawed. both are trying improve themselves. Each story arc moves their relationship on a little bit more. The supporting cast all add to the depth.
The story IS slow, and the theme puts a lot off (I had no interest in S&M when I started reading this; still don't), but it really is a must read.
Oh and unlike the previous reviewer, I think the pair are slowly spiraling towards each other, and they inevitably will end up together.