My review: The approach is outstanding, the execution is lacking in key area and great in others.
7 of 10 - I'd watch again, but I wish they'd done some things differently. They needed to spend more time connecting us to the various characters before throwing them into peril, and then they needed to ground the awesome array of sci-fi / comic elements they introduce better so that they make sense within the world they occupy. This feels like they needed another 6 months to cook before filming and another $50M in the budget if they wanted this to be their centerpiece.
Feel: The comparisons to the Animated Justice League series are well earned. This is a lived in world with Super Heroes that is comfortable being a Super Hero world in a way that Marvel had to edge into slowly. I think the feel of the world works well.
Where the feel fails is where the sci-fi elements are introduced, but not respected. They drop them in without thinking about the ramifications of having them in the movie. In other words, they do not have the setting and story react to the presence of the sci-fi elements ... and thus the elements do not feel like they are in equilibrium.
Characterizations: The casting is generally good and hits unique tones for some characters (all the parents of Superman) that we have not seen before but that make sense and work for the story. Lois is perfect. The biggest drawback is that we get a slew of characters in this movie and don't have any reason to be attached to them other than that we know their name and alt histories. Jimmy Olsen plays a significant role in the movie, but if his name was not Jimmy Olsen we'd have no reason to care about him. We can easily argue that we all know the rough idea of these characters and didn't need to spend time getting to know them ... but I think we didn't spend enough time building up our attachment to these iterations of the characters. That left me feeling unconnected to many of their adventures.
Superman himself has the right ideas, but doesn't quite nail the humble moral superiority of the character. He struggles to be the icon rather than being the icon. He isn't comfortable with himself where we're used to Superman characterizations that stand above and inspire with their moral certainty. He needed to be less defensive about his beliefs - being dynamic and strong in them up and until a point where someone convinces him he is wrong. He can flip flop in his beliefs ... but Superman can't be defensive and seem like he is unsure if he is righteous.
You can feel the James Gunn and it works - but it needed more refinement.
Action: Generally ok, but Snyderverse captured Superman's capability better. This Superman feels too small for Superman. His Kaiju battle is fine, but doesn't make sense in terms of the damage inflicted or the tactics used. The scene where he relaxes while a threat is being battled by others just feels wrong. He never feels like he has the speed of the Flash or that he is the strongest being alive. They call him the most powerful Metahuman ... but he feels like one of the crowd for most of the movie.
Sci-fi: They embrace the sci-fi of a comic book world that feels ripped from the pages ... but the implementation, like the characterization, feels a bit flat. They introduce ideas that needed to be refined before being implemented. There is significant action taking place in a sci-fi setting, but the setting doesn't seem to make a lot of sense. There is a prison with people in essentially glass boxes ... but how do they eat, breath, use the toilet, etc...?
What I've had done differently: I'd have put about 30 more minutes into the movie that ground the characters - especially the Kents and the Daily Planet team. I'd have cut a few of the Planet staff members from the story - they would better be saved for another day. The grounding might be an expansion of the scene where the Planet team discusses the actions of Superman. It could have been a debate where characters showed you their beliefs and ideals rather than a bunch of criticism delivered too quickly. Giving that scene time to breath could have been a huge improvement. I'd also have either augmented or removed the Ultraman storyline. It needed more support to work well. There were two or three directions from the comics they could have taken it that would have worked but that they failed to connect. It just felt ... wasted.
I'd also have considered adding two or three more people to the Justice Gang. Three members made it feel like the Deadpool jokes about the X-men appearing in Deadpool. If they'd tossed in Red Tornado, Elongated Man, and Zatanna it would have gone a long way. These characters did not need to be fleshed out (and only Mister Terrific really gets any character in the filmed version), but it feels like they were cheap where the story would have been better with more substance. They also severely under utilized the Green Lantern powers - which worries me for the Green Lantern series.