I just saw this. It's hard to say how someone unfamiliar with the source material would take the movie. It has some very compelling spots, and Saoirse Ronan is a riveting screen presence, but I don't think it quite works overall. A friend who saw it with me, and who had read only the beginning of the book, was lukewarm on the film.
It feels very much like in trying to jam in almost all the book's plot points, Jackson ended up shortchanging all of them. The plot progression feels choppy and the characters underdeveloped. Major events and plot points from the novel are often left in without any of the preceding development or context, so I was left wondering whether they'd make sense - let alone have any emotional impact - for the average viewer.
The best example is the relationship triangle between Susie, Ruth, and Ray; the big payoff scene (light spoilers to follow) is replicated pretty closely in the movie, but it makes little sense because Ruth and Ray have both been reduced to cameos prior to this and no indication is given that they've built any friendship around their shared loss. The stakes of Susie's decision about how to use her time on earth are also not made clear - the love scene (reduced to a kiss in the film) is intercut with a scene of Susie's body being disposed of, which doesn't adequately convey the choice she's made to let go of the investigation. My friend who hadn't read the book was baffled by what the hell was happening.
The editing problems also come to the fore here - the ending is, by pretty much any standard, jumbled and confusing. Several scenes from the book are mashed together to form a climax, which I actually like the idea of but is hard to follow in practice. Jackson tries to restate the novel's "pushing for closure vs letting go" theme by having Lindsay escape with the evidence (made more of an explicit smoking gun in the film than in the book) from Harvey's house and then elect NOT to show it to her father. But then she shows it to her grandmother instead, at which point we want resolution on what the hell happened to Harvey being brought to justice.
This is where things go off the skids. We're shown a literally blink-and-you-miss-it shot of police sirens blaring, Harvey goes to the sinkhole to dispose of the body, we get our Ruth-Ray scene, and then - with no concrete indication that any time has elapsed but, confusingly, what looks like some subtle aging makeup on Harvey - he's standing at a bus stop and we have, as in the novel, his final scene with the icicle. This is made even more confounding by the fact that Jackson cast a girl who looks nigh-indistinguishable from Ruth's actress for the little role in the icicle denouement.
The result is that my friend thought that Harvey walked straight out of his house, Ruth chased him, and then he fell off a cliff for no discernible reason (Jackson tries valiantly but fails to translate into visuals the book's clever foreshadowing of Susie's death-by-icicle thinking). It's all a bit of a mess.
This all happens VERY QUICKLY, intercut with the family hugging and reuniting and Lindsay inexplicably pregnant all of a sudden. This has zero emotional weight because the family struggle has been pathetically underdeveloped and we're given almost no evidence of Lindsay having a meaningful relationship. It also doesn't make logical sense because the way the film is edited affords us no clear sense of chronology - we have no idea how old anyone is at any given point, or how much time has elapsed between scenes. My friend hadn't the faintest inkling that more than a year had gone by and assumed Lindsay was still 14 until the out-of-nowhere pregnancy shot during the denouement.
Brian Eno's ethereal, synthy score is at times very effective and moving but weirdly inconsistent and intrusive at others, especially during 'action' moments, where he resorts to blaring and inappropriate electric guitars on several occasions.
The much-criticized ceaseless procession of effects shots is indeed needless and distracting at times, but the movie has bigger problems. It's handsomely made and has a uniformly excellent cast anchored by a captivating lead performance. It just doesn't hang together in the end.
