This post contains spoilers for Justice League and the other DC Universe films.
As a single movie, Justice League manages to keep a brisk pace and a single focus. The film sees the return of Zack Snyder to his DC Extended Universe, continuing the story he began in 2013's Man of Steel and exacerbated with 2016's Batman V. Superman: Dawn of Justice. In Justice League, Batman (Ben Affleck) and Wonder Woman (Gal Godot) have to gather three previously-seen-in-mp4-video metahumans (Jason Mamoa's Aquaman, Ezra Miller's Flash, and Ray Fisher's Cyborg) to form the Justice League, and save the world from Steppenwolf (Computer/Ciaran Hinds), who wants to use the power of the alien Mother Boxes to turn Earth into a living Hell. As of the publication of this piece, reviews aren't kind; the movie currently commands a 40% on Rotten Tomatoes, a number that puts it above Batman v Superman's 27%, but still in the "rotten" category.
But in one way -- one essential way -- the movie is a success.
As comic book mega-property, the DCEU has always felt reactive. Unlike Marvel's decade-long plan of interconnected movies, where consistency is the prioritized perk, DC's universe-building (overseen by Warner Bros. Pictures) seems to evaluate each installment based on fan and critical response. If something needs to be "fixed," as many trade reports have detailed over the years, creatives swoop in to do what they can before the next release date. Llike a slow-moving tanker evading an incoming calamity, they do what they can. The one good thing about Justice League: the shift is totally successful, with the powers that be pivoting the DCEU out of grimdark, deconstructionist misery. Bluntly put: the movie is fun.