SHORT TAKE:
Justice League is an amusing and entertaining excuse to unite the DC comic book characters into their version of Marvel's Avengers but requires some parental supervision because of two short but poorly chosen off hand political and anti-Christian polemics which should require discussion between younger viewers and their parents.
LONG TAKE:
Writing an origin story isn’t easy. You have to deal with a lot of exposition and expectation all while trying to find a new way to tell an established or sometimes even cliched story line. Sometimes it works spectacularly well – 
like the Chris Reeves’ Superman or 
this year’s Wonder Woman. Sometimes not so well,
like Wolverine or 
Eric Bana’s Hulk. And when you’re trying to do a team effort that issue becomes exponentially more difficult.
Such is the challenge facing Justice League, especially when three of the characters have only appeared in cameos in Batman V Superman: 
Flash/Barry Allen (Ezra "Fantastic Beasts" Miller), 
Cyborg/Victor Stone (Ray Fisher), and 
Aquaman/Arthur Curry (Jason "Drogo from Game of Thrones" Momoa).
The premise is that Bruce Wayne/Batman, in the wake of Superman’s untimely demise, has discovered that the void left by his super-colleague is attracting 
very ugly aliens called parademons who look an awful lot like 
larger flying version of the dwergers from Hugh Jackman’s Van Helsing.
To prepare for the invasion by the parademons master, 
Steppenwolf (super villain, not the ‘60's band) who seeks to remake the world in his own image, Bruce and Wonder Woman/Diana set out to recruit other supers. 
Bruce to seek the Flash and 
Aquaman, 
Diana to convince Cyborg. All three of the newbies have troubled pasts (Cyborg, Flash and Aquaman can put their baggage in the closet with the steam trunks from the traumatized Batman and grieving Wonder Woman) but eventually agree, only to find out that their considerable combined efforts will not be quite enough to even create a stalemate with Steppenwolf.
A nice little theme of not putting your light under a bushel, of being able to overcome your fears of failure or worse, dread of the responsibility in leadership, is interwoven in the storyline, but only as light embroidery, not as sustaining warp and woof of this cinematic costume quality fabric.
Other reviewers have complained the story felt disjointed and somewhat disconnected. By necessity this is what happens when you try to introduce three major players into a five "man" mix with a story intended to propel them together and still try to keep the movie less than 15 hours long. A few things have to be cut and you have to edit down a bit.
While Justice League isn’t as lighthearted as 
Guardians of the Galaxy nor near the apex of the genre that is 
The Avengers, or 
Wonder Woman, it’s an engaging enough flick. And isn’t that good enough?! I mean, come ON, it’s a live action comic book!! If you want Shakespeare then you could watch…..well, OK 
Branagh’s Thor….
But seriously, there are at least four compelling reasons why I liked Justice League and can forgive them a lot of plot and presentation weaknesses because of them:
1. 
We get the return of Wonder Woman in a vehicle which did not require a three year wait as many other sequels are glacially cranked out. It was very nice to see Gal Godot don the Amazonian one piece and watch as she balletically beats up bad guys again.
2. DC has managed, as with Wonder Woman, another transformation I would never have thought possible. 
As a kid Aquaman was on the top of the list for the lamest of super heroes. Mostly a Ken doll who could hold his breath for a very long time he didn’t even make a blip in my Superman-loving radar.
But in Justice League 
Aquaman is a whiskey swilling, tatooed, long haired Norwegian-ey good ole boy who looks like a wrestler and acts like a rock star. This guy is just fun to watch 
as he exudes the kind of joy battling parademons we haven’t seen since 
Woody Harrelson’s delightful walking dead killing spree in Zombieland.
3. We see the return, albeit in cameos, of 
Jeremy irons as Alfred, 
JK Simmons as Commissioner Gordon, and Cyborg's father 
Silas Stone played by Joe Morton, whose pedigree with sci fi dates back a lot further than you might think – through a stint on Warehouse 13, to a key role in the 
Terminator franchise all the way back to 1984's quirky sci fi indie 
Brother from Another Planet.
4. And most importantly, which reason would have been enough to get me to see this movie all by itself……..well……I’m not going to tell you, but you’ll know it when you see it.
That being said, there are also three reasons I have to take exception to, which are largely unrelated or at least unnecessary to the story or comic book characters per se:
1. GLOBAL WARMING PROPAGANDA:
Bruce shoehorns a throw away comment to Aquaman about global warming as though it is an established fact rather than the cock and bull fantasy of environmental wackos who want an excuse to tyrannically limit First World progress into the 21st century in order to feather their own Al Gore-jet flying coffers.
2. ANTI-CHRISTIAN CHEAP SHOT:
Lex Luther takes a gratuitous stab at Christianity which I, for one, didn’t appreciate. Granted, he is a bad guy, but his comment stepped right over offensive into blasphemy. When a statement is so over the top that it shocks you out of the suspension of disbelief it does nothing to promote the storytelling either. It neither served the plot or character well nor will endear it to any Christian audience members, and was just plain rude. I continue to be annoyed by the singular targeting of the Judeo-Christian faith by a Hollywood which used to produce movies like Going My Way and Schindler’s List.
3. As I have alluded to in the earlier part of this post there is an awful lot of derivatives, echoes and dopplegangers between the two rival comic book universes DC and Marvel. Along with the ones I have already mentioned like parademons, there is 
Steppenwolf (DC comics) who looks a LOT like a combination of the fire demon 
Surtur who appears at the beginning of Thor: Ragnarok (Marvel comics) and 
Thanos from Guardians of the Galaxy (Marvel). Further Steppenwolf’s goal is to bring together three 
"mother boxes" (DC) which are not explained well but when glowing look like red versions of 
the blue Tesseract from The Avengers (Marvel), and when joined will reshape the Earth with tentacle transformers made of Element X (DC) which look an awful lot like the tentacles from Thor: The Dark World set loose by the Dark Elf Malekith from the Aether (Marvel).
See what I mean? After a while they all kind of blend together. In addition, the are a LOT of other counterparts in each world. I have made a short list below of the most noticeable ones. (And the years they first appeared in order to put to rest any debate about who predated whom. In short – DC and Marvel took turns being "first".)
For the most part Justice League is an airy simple romp. Lots of cartoon violence, super powered heroics, and over the top demonic bad guys. Bringing the "old band back" is a hoot to watch and like the first Star Trek movie, or the first waffle off the griddle, they are entitled to "warm the pan up," so to speak, for future efforts. Besides, they should be able to pull off pretty much anything now that they have Wonder Woman and………you'll see.