For the bad: it's not original, it's not faithful to the characters, and it has incredibly bad writing and inconsistencies.
The idea of Joker bombing Metropolis and killing Jimmy, Lois etc. has been already done about 26 years before. Superman #9 (1987) has literally the same plot of Injustice, with the difference being that Joker was stopped.
But even if he didn't get stopped we see another injustice scenario in Superman Annual #3 (1991), Intergang nuked Metropolis killing everyone including Lois, obviously, Superman become a tad more aggressive, but no psychopath child-killer.
And then we got Kingdom Come, one of the best DC comic books ever, in this story Joker kills everyone at the Daily Planet, despite this, Superman "defended" Joker after Magog killed him, again, no psychopath child-killer.
This comic is a victim of what i call "Batmanization", in short, since around the 2000s/90s, Batman become DC best selling hero by far, and as such, they find ways to make him stand out from everyone, this translated in every super hero getting shit on by him, this transformed Batman in a cheap iron man knockoff (imo)
For the good: It made DC a lot of money?