The ending kind of lets it down (Spoilers, obviously)
This is a very solid film (and I'll watch anything with Jennifer Connelly in it) but it definitely needs a stronger ending. The way Dahlia just kind of follows the ghost upstairs and finds the body in the water tank is anti-climactic after watching the whole mystery unfold. And there's no real buildup to the attack in the bathtub and the sacrificial death scene that follows it - in fact it kind of comes out of left field, and feels tacked on.
Oh, and just to be extra nitpicky - how can Dahlia be thirty years old if she was a little girl in 1974, and the film is set in 2005?
Other than those niggles, it's a mighty good flick.