Honestly, I'm surprised at how harsh some critics have been on this movie. The fans seem to enjoy so far, and they're the real critics when it comes to superhero movies.
Justice League is far from being a perfect movie-- it isn't mind-blowingly spectacular, the CGI is often mediocre, and it feels a little choppy, a result of Joss Whedon filling in for Zach Snyder near the end of the production & reshooting parts of the film. But it's extremely fun to watch-- a complaint many people had about Batman v Superman-- and it gets the characters right. Jason Momoa rocks as Aquaman, Ezra Miller makes for a comical, yet socially awkward new Flash, and Gal Gadot continues to nail Wonder Woman. More importantly, it gives fans reason to be excited for future DC movies like Aquaman, Flashpoint & Green Lantern Corps.
The DCEU is still a work in progress. After the huge success of Wonder Woman, Justice League needed to build off of its momentum, in order to get the DC train up and running. It certainly does that, but certainly not as well as it could have done. It may be time for Warner Bros to part ways with Snyder, and hand the wheel over Whedon & Geoff Johns, in shaping this movie universe.