I have friends in the industry that do practical special effects / makeup, they heard from someone they trust, take with salt and going on third hand here blah blah blah. This isn't from the test screening but it should be close.
The movie in its current state is a mess and not that great. Its serviceable but only just. If you like big dumb action films and let them slide when it comes to plot, you will be ok.
Positive Comments:
They nailed Superman's tone in a way they haven't live action since 1980s Superman II. There is way too little superman in the film but that might be fore the best. (the right amount of a good thing) Aquaman is really good. Wonder Woman has a few great moments but is mostly in the background. Parts of the new soundtrack are great.
Neutral:
The Flash isn't as good as the TV show version but it might come down to the movie is too busy for the character to develop. Same with Cyborg.
Problems:
People are going to hate Batman. The movie leans on Batman for the comedic parts and it's awkward. There is a moment (and I haven't been told what this moment is) that channels the Adam West Batman. If it stays in the film, Affleck will probably regret accept the role and some feel, would sink the film. There is a lot of pull to leave it in because of Adam West's passing. There is hope it will be cut and they will do a dedication credit instead. Even with that moment removed, his moments in the film aren't good.
Coming from special effect people, they feel the effects could have been so much better. There was too much time when money was spent and nobody was in charge of the film and it's too late to fix. Most of the concerns come from the villains.