I have mixed feelings about it myself; was expecting better from Wan. Not that it was bad, per se, but the way it was hyped up to be the project that would surpass the Saw and Conjuring entries was a bit overzealous (and, in retrospect, inaccurate) IMO. There are still many aspects of the plot I don't understand, and while the "twist" involving the graphic body horror element was interesting, I just felt like most of the film was in the paint-by-numbers style of other forgettable horror projects -- notably the beginning parts (you know....a sinister shadowy figure emerges from dark corners to avenge some family member while a female lead is made to feel that she's making all this crap up in her mind....we've seen this countless times before).
Also, all the reviews I read before going into this mentioned the dollops of humor Wan poured on, almost making it seem like it was in the likeness of classic 80s comedy-horror such as Return of the Living Dead; to me, there wasn't really anything to chuckle at. Reviews also pointed to the potty mouth exuded by the female detective character, and that this was an example of the aforementioned "humor" -- but I just didn't see it that way.