Ryan Swen’s review published on Letterboxd:
A fundamentally frustrating film in its approach and in its waste of the runtime. Simply put, this film would have made for a very good hour-long film (for spoilers reasons I'll go into in the final paragraphs). This is an uneven film, a film where the good and the inexplicably awful coexist, sometimes in the same scene or the same shot, an issue which can be attributed to both the mixed script and Abrahamson's pedestrian direction.
Larson is without a doubt the best part of this movie, even though she is just good, remaining committed to her character and her situation in some admittedly interesting ways. Tremblay is also good, but in too many situations he has a tendency to overact in a screaming manner that becomes rather grating. Together, they do manage to portray a loving and somewhat realistic-feeling mother-and-son relationship.
*POSSIBLE SPOILERS FOR THE REST OF THE REVIEW*
The first half of this film is surprisingly good, plain and simple. The pacing (at least when I didn't expect the film to go on after the characters escaped Room) felt right and consistently engaging, though there are some grating parts when Tremblay narrates and/or throws tantrums. Unfortunately, the second half absolutely tanks the film.
The second half is utterly superfluous, as it says nothing about Joy and Jack and their situation that has already been said. The film should have ended right after the two had reunited, but instead the film just plods along and ends rather anticlimactically, though the end of the ending is somewhat okay.
Additionally, those who say that this is told through Jack's point of view (hence excusing the bad narration) aren't entirely correct; there are more than a few moments where it is told from Joy's point of view, which is made blatantly clear when the camera moves outside of the wardrobe.