Happiness' first few volumes are great; Aku no Hana with vampires, what's not to like? Unfortunately, things spiral out of control pretty quickly, as this spooky Let the Right One In-esque romance-horror becomes a Tokyo Ghoul/Ajin wannabe around chapter 15 or so. We're left with 35 chapters of shady government agencies, cultists, a villain straight out of a Dean Koontz bargain bin book, two semi-interesting characters getting sidelined for what feels like forever, one uninteresting character getting mutilated for what feels like forever, and time skips. Lots and lots of time skips.
It's all so...typical. A damn shame considering the manga's early promise and the mangaka's track record for non-commercial weirdness. I guess if there's a lesson to be learned it's this: selling out may lead to large amounts of cash, women and fame, but you're gonna get a lukewarm internet review from an unemployed thirty year old who still reads comic books, and to win him back you'll have to write incest-horror (see the excellent Chi no Wadachi).
A final note: the art is fantastic. Just look at that cover. There's also quite a bit of Van Gogh and Edvard Munch inspired panels scattered throughout. Alas, the rest of the material is not nearly up to the art's high standard; you're probably better off doing a google image search for the best bits and skipping the rest.