Hmm, where to start? Well, I've read the series up to chapter 35, so all my comments are based on what I've read so far. I will also say that when I begin reading a series, I read everything that's been released up to that point, unless a work is just so absolutely terrible that I'd rather burn it (not the case with this series, though). Also of note, I just finished reading Kurohime; whether or not that may have influenced my feelings about this series is left up to you.
When I read this series, I never feel 'romance in the air,' in a traditional sense at least, and I may be wrong for anticipating that, but it really is irritating. I may just be put off by the strangely sadomasochistic interaction between the two main characters; perhaps I'm unconsciously rejecting the female leads' complete control over the duo's relationship.
So, lets start the QQ'ing. The main characters are taking it far too slow for a relationship that started in such a purely physical way, and I find that to be counterintuitive and glaringly hypocritical of the mangaka and the two leads. Quite frankly, the prevalence of drool is a bit gross, but it doesn't detract from the story for being 'gross'. The fact that nearly all emotional interaction between the two leads is done via a physical means (drool), is an interesting concept, but in the end I feel that it is glorified fanservice. Admittedly, the fact that the mangaka's managed to make the fanservice serve a purpose within the series itself is laudable.
I was able to get into the story, but I'm able to do that with nearly every novel/manga/etc that I come across, so that doesn't mean a whole lot. But unlike other series, I never really 'liked' any of the characters a whole hell of a lot. Although the mangaka stays true to the name 'mysterious', he could have done a lot more exposition on either character until now. I find the female lead's aversion to letting the male lead touch her in any way, to be more disappointing, hypocritical, and selfish than it is interesting or mysterious. Asking your girlfriend for a hug? That's something I just couldn't wrap my head around, no matter how I tried, so cheers if you're into that kind of thing.
The series definitely improves in the later releases, but by this time, it was too late for me to retain a whole lot of enthusiasm.
The artwork leaves much to be desired; I've seen far better artwork in shounen works, let alone seinen works. What I'm saying is that the mangaka's drawing style is weirder than Eichoro Oda's in One Piece. If you couldn't wrap your head around that series' art, don't bother with this one, it'll just irritate you.
I only started reading this series since it was rated highly and because almost all the comments are positive, but I didn't really enjoy it. If you want a lot of fuzzy feelings, lulz, or a realistic romantic series (and no drool), look elsewhere.
This review is purely subjective and haphazardly constructed, but I hope it helps some of you make up your mind.