Let's make one thing clear, the phrase "it's a movie for fans", is not an argument, let's go to the point.
Director Zack Snyder confirms here that he is a bad director, he doesn't know how to tell stories, he is so desperate to introduce the Justice League that the movie ended up being totally overstuffed and nonsense at some point. The montage of the movie is just HORRIBLE, like... one scene there's some tense moment and then suddenly, the screen turns to black and cut to a stupid scene of Lois Lane investigating something or Diana Prince checking her e-mail. Bruce Wayne's nightmare sequences are just empty, stupid and pointless.
The dark tone of the film is not necessarily bad, is not meant to be a joyfull film.
I still don't understand WHAT THE F*** is Laurence Fishburne doing in this movie, basically the same thing in "Man of Steel", nothing.
The movie it's so rushed for a 2 hours and 40 minutes that the epic battle between the 2 greatest heroes ends up being nothing but disappointing, and almost laughable at the end. Transposing scenes from the comics to the big screen is cool, but where's the context on them ? it's a buy fan ticket.

Doomsday........... Horrible, terrible, a visual rape, an insult to the cinema.
Lex Luthor................ no words worth it.
Wonder Woman: 5 minutes in the film and 5 as Diana Prince. But Gadot does a good job
Superman, it's actually better than in the previous film.
Batman is the best thing from this film, a bloodthirsty dark knight who can finally move his neck. It's the batman i've always wanted to see in the movies. Nice work Ben Affleck.
The other good thing from this film is Hans Zimmer's glorious score... Hans Zimmer is the god from this movie.

Conclusion: Could be the best superhero movie of all time, but ends up being worst as many of.