It is a strength because of the unconventional relationship between the two leads, Tiffany (played by Jennifer Lawrence) and Pat (Bradley Cooper). They are both suffering from various mental disorders, Pat is bipolar and Tiffany is neurotic and depressed and dealing with the death of her husband. They are both difficult people, both obsessed with their exes in different ways.
Pat agrees to help Tiffany with a dance contest she wants to enter in exchange for her help to get a message to his ex-wife, who will not speak to him. And so their relationship stumbles on, dance practice mixed up with Pat's father's obsession with the Philadelphia Eagles, Pat's psychiatrist, his over-achieving brother, his attempts to be a useful, reliable human being, and Tiffany's desire to hold things together. They are trying so hard, both of them, to hang on to some kind of sanity, it is difficult not to feel for them both, to be on their side. The longer the movie goes on the more you want things to work out for them.
Yet this is also the film's weakness. The ending is too pat, too easy. The fact that they are both supposed to be in some kind of mental anguish becomes less and less relevant the more the film goes on, until it seems like they are simply getting better on their own, as if depression or bipolar disorder are something like the common cold. Cooper's character Pat is severely distressed in the beginning of the film, and yet by the end he has quickly become almost well adjusted. The ending is a bit of a cop out, it smoothes over what cannot, in reality, be smoothed over, and the resolution comes with too few complications to be believable.
Still, it is very enjoyable. It has feel-good moments, truly hilarious scenes, De Niro, as Pat's father, is back to a role he can do something with, and it has characters it is impossible not to care about. And two things stand out and make the film worth watching. The first is the dance competition near the end. The film has been building to this, and the explosion of joy as the results are announced is worth seeing, and has all the more impact because of the complications of a bet and the presence of Pat's ex in the audience. It is a memorable scene.
The second is Jennifer Lawrence. She is truly mesmerising in this film, when she becomes a central figure the story really picks up, and she pretty much dominates any scene that she is in, including ones with De Niro. She won the Oscar for this, and for once they got that decision right, this quickly becomes her film. She is stunning in her intensity and vulnerability and complexity. It is worth seeing just for her performance.