My opinion:
It is not top notch Fulci and it is even more incoherent than his other works of supernatural surrealism. The acting is quite dodgy and it seems rather rushed and less carefully constructed than his best films.
But it is tremendous fun. It has some lovely trippy oneiric scenes that are rewarding and it does keep you interested. The Shameless print is wonderful (especially if you grew up with the fuzzy pan'n'scan UK VHS titled "Possessed"!).
So, yes, it is well worth buying!