Nowhere near the disaster that some are saying!
I'm not a gamer, but I am a fan of the Injustice comics, and I knew going in that a lot of Injustice fans disliked this movie. But having readied myself for the worst, I found I actually liked it. The voice cast - led by Justin Hartley (Smallville) as Superman, and Anson Mount as Batman - do a fine job. Harley Quinn (voiced by Gillian Jacobs - sounding uncannily like both Arleen Sorkin and Tara Strong) made me laugh out loud more than once. The animation style isn't my favourite (it's similar to the DCAMU/'New 52 movies'), but that's just personal preference - it certainly didn't distract me. The writing is fine so far as dialogue goes. As for plot, that seems to be where a lot of the hate lies...
The Injustice comics ran from November 2013 to February 2021. This movie runs 1hr 18mins. There's no way this was ever going to take more than a few elements from the original saga. It contains some aspects of Year One - the Superman/Joker/Lois set-up, the immediate fall-out from that - but then becomes largely its own thing. This inevitably means that some characters get less screentime than hoped for, or are cut all together*. But working within the limitations of the runtime it does a pretty good job. It's probably easiest to think of this as an alternate version of Injustice, rather than a direct adaptation. 7.5/10
*Incidentally, crowd scenes on the Watchtower showed the biggest gathering of League members I remember seeing onscreen since Justice League Unlimited (I could actually see movie as an alternate future to the DCAU of another Earth).