If this had been released as a series of 6 45-minute episodes, it would have been a solid limited series. It's not so solid as a single feature film. For a 4 hour movie, the pacing is terrible. The only reason this works is we're watching it at home, and we can pause it, get up, get a drink, go to the bathroom, etc.
The story is definitely more complete than the theatrical cut, and gives us a much better hook into the two characters who still haven't had their own films. It also sets up a Flash movie much, much better than the theatrical release did.
Snyder does an interesting job of taking us from the narrative we expect—centered on Bruce's efforts after Clark's death—and weaving in the story of Victor Stone. In the same way BvS was a story of a Batman who's been in the trenches for too long, ZSJL is the story of what happened to Victor Stone, and how he's trying to deal with it as he moves forward.
That said, the film still suffers from the DCEU's 'we have to play catch-up with the Avengers' obsession. There are a lot of elements in it that might have been better served as the centerpieces of their own movies. Separating them out would also have allowed those movies to precede Justice League, and the film could have been much, much tighter as a result.
Much of that, however, is squarely at the feet of the studio, and Snyder's work here is stronger than the half-finished-then-reshuffled effort WB released into theaters.