While the film contains most of the narrative issues the book had, some fans of the book, and those who only saw the trailer, may like the film.

Rating: 7/10 (6 September 2014)

With reading the book this is based off of, as well as the book's sequel, my expectations were heightened since I knew everything that was supposed to happen. Though, as I learned from past book to movie adaptations, getting your hopes high for a movie adaptation is a foolish thing to do. For even if you like the cast, there is always the issue of what was kept, or changed, for the book's transition to the big screen. Leading to us talking about If I Stay.
Characters & Story
Two people living in different parts of the music world, one rock and one classical, one day happen come together. It begins with Adam (Jamie Blackley) hearing Mia (Chloe Moretz) play her cello, and watching her passionately go at it. From there he asks her out and their relationship begins. We watch the socially awkward Mia fall in love with this guy who is on the way to becoming a rock god, while her retired Rock & Roll parents, Kat (Mireille Enos) and Denny (Joshua Leonard), watch in delight.
However, no story dealing with romance can be without conflict. Which, for If I Stay, mostly deals with the fact Mia is younger than Adam and at a different place in her music career. But that is just her issues within their relationship, seen in flashbacks. As for the real conflict which may drive them apart, it is a car accident in which Mia's family seemingly are dead, and she is wondering if, without them, should she stay?
Praise
As Moretz grows as an actress, and a bit more consistently brings depth to her roles, you can see from movie to movie that she is really honing her craft. For, as seen in Kick Ass 2 and Carrie, as much as she can play a smart mouthed young lady, she can also draw you in with her character being vulnerable, having a sense of uncertainty, and Moretz seeming just as much ready to fight and seem bad ass, as she could breakdown into tears.
As for Blackley as Adam, while I admit I don't see him as a good fit for the roll, since he seems more geared to swoon the audience than be my perception of Adam, it is hard to deny Blackley and Moretz play don't well off each other. They help create the visuals for a love story in which while he helps Mia bloom, she doesn't fully lose herself and while she may fall in love, her insecurities don't just abate because of how powerful the love Adam presents is.
Leaving one last thing to note: In the film there are great classical pieces used, and Adam's band is given songs to perform of which more than half were actually good.
Criticism
Unfortunately, between the transition from book to screen, a lot of the issues I had with the book's narrative of going back and forth on a fairly constant basis were not changed. Which sucked to me since those jumps in time make it hard to not only stay connected with the story, but also emotionally connected to the characters.
Though the real problems with the film deal with changes made due to artistic license, omission, or necessity. Such as Adam's band not being called "Shooting Star"; a lot of Adam trying to get into the ICU to see Mia being cut; the film trying to instill a bit more hope in Mia's family surviving the car crash than there is in the book, as well as more drama in Adam and Mia's relationship; and then there are general cuts you recognize had to be done. Things like us seeing how Mia and Kim became friends; seeing Kim's mom; the lead up to Mia's grandpa taking her to her Julliard audition; and a lot of the back stories including Willow's and her grandparents.
All of which sort of take away the impact of dramatic moments to me. For with grandpa and Willow's roles rewritten to the point they are, their appearance feels like it means less. Like with grandpa, all the build to him taking Mia to the audition is wiped, and all that was saved is him talking about him not supporting Denny and Denny's decision to leave the band for Mia. Then, when it comes to Willow, honestly it felt like they should have wiped her character. For with her not being a force to reckon with when it comes to Adam seeing Mia, nor Teddy being at another hospital, making it where her appearance could truly be devastating, the film loses most of its drama to bad screenplay decisions.
Leading back to my issue with Blackley. He, to me, doesn't look like Adam, and also has the same issue Gus, in The Fault in Our Stars, had where it seemed he was more so there to make girls squeal than woo Mia. Leaving us to talk about Moretz: in the film, I felt she over acted. For, whether I just saw Mia acting differently in the book, or her performance just not being to my taste, I felt she was a bit too over dramatic, even considering the circumstances.
Overall: TV Viewing
As someone who read the book, I honestly want to say to skip this. A lot was changed, or omitted, which doesn't make this something worth seeing. However, seeing this from the point of view of someone who didn't read the book, I think there are enough laughs and cute moments throughout the film to justify seeing this. Leading to me labeling this as "TV Viewing" since it will likely disappoint the book readers, but seems like it may be OK for those who never touched the source material.