Just took a look at this film again. It's battered and bruised, a lot of the scare moments just look silly, and the acting is pitched just this side of hysterical (with maybe Rosanna Ortiz crossing the line a half dozen times).
But about an hour and fifteen minutes into the film, when the action moves to the vacation house, it starts to become great. Celso uses silence, uses shadows and there are moments here that recall Hitchcock (Walters' ultimate fate, the little slashes made at Roces), and moments that recall Gerardo de Leon (the apparition popping up out of nowhere).
Most impressive is Roces' climactic soliloquy, when she dares Ortiz to come out and face her, pit Ortiz' love against hers. You realize that the problem is Ortiz all along, and her all-consuming, jealous love which is more obsession than love. Ortiz might be likened to the Furies, which harry the object of their anger to death without actually killing him or her. Not a perfect movie, but a substantial and (at least for long stretches) extremely well-made one.