I liked it. Similar premise to Watson's Before I go to Sleep, but more artfully written. The characters were all real in that they were grey (and not just black or white); they were all complex and (with the exception of Jonathan) I liked parts of them all.
I did not like that all of them came together. It seemed to contrived and convenient that they would merge in LONDON (hello, very big city) and have similar psychoses (I think both Mr. Early and Freddie have issues with leaving their home, clearly Charlotte and Freddie are both averse to work). I think some of the point of the novel is that we are all a bit alike (even though we can be vastly different), but the links between all these folks were too strong given that they started the novel as strangers.
I loved the sex scenes between Hazel and Jonathan. They were certainly creepy, but nicely done (not overblown and over the top, just matter-of-fact from Jonathan's viewpoint).
Mr. Early's comment: "helping. Not my subject. Can one person ever help another? Often I think the answer is a resounding no" parallels my thinking on the matter. Frequently people wait to be rescued or hope to save another (or convince them to cease self-abusive behaviors) when really we can only make choices regarding ourselves. Of course, this is the echoed moral of the novel. None of us can save another (even alas, poor Freddie cannot rescue Hazel), but we must be left to make our own choices and live with the results.
Overall it was compelling, thoughtful and well written. Certainly worth the time.