I didn't go in with high expectations but I was pretty surprised at how bad the movie was. The story seemed to have no central plot, the team gets together at the very last minute, the climax falls flat, the humor fails, and the actors have no real chemistry and most aside from Robbie and McGregor turn in bad performances.
Spoilers ahead:
The movie starts off alright introducing Harley's breakup (even if its a bit indulgent), but very quickly the introductions of the characters are muddled and diluted in the constant rewind back-and-forth. I could see they were trying to be clever or creative but it just sucked the momentum out of the plot. I was also a bit confused early on whether to take the movie seriously or not. Some of the emotional beats for Harley are supposed to be genuine, but so much of the danger seems trivial.
The biggest fail for me is that the characters are hardly fleshed out outside of their context in the climax. Black Mask is torturing some guy for no reason and his backstory is purely exposition, as is Montoya's and Huntress's. Black Canary's situation as a singer down on her luck didn't really strike a chord with me, and her confrontation with Montoya about her mother sounded pretty cliche.
One place I thought the movie really started falling apart was the scene with the Taiwanese storeowner (who calls her Lotus Flower...wtf), in terms of both editing and story. In a movie already stuffed with shallow introductions for supporting characters and trivial henchmen, it was hard to buy that he was anyone actually important to her. Later, once Huntress shows up, the scene cuts away right as the storeowner asks who she's looking for...yet Huntress doesn't find Harley then, so what happened? After Harley has to escape from Black Mask goons, and not Huntress, the storeowner reveals that he betrayed her because it's "just business", and it's supposed to be this big emotional moment for Harley that just doesn't ring true because this dude is random as hell. Then we suddenly see Huntress in the last shot of the scene where BM is lashing out at a club guest, even I never remember it being established that she was there to begin with.
Also, I thought McGregor did a decent job at capturing Black Mask's manic style, but the way they shot him was weird. He would yell "Woo!" out of nowhere with nobody else in the shot to interact with, and sometimes he'd speak and the camera pans away from him while he's still speaking. When he first instructs Canary and Zsas to get the diamond, they speak at the same time and it comes out muddled. Just weird editing.
Once the girls actually teams up (just minutes before the third act concludes), none of the chemistry works. Huntress's deadpan was bad, Canary's humor was awkward. I thought Perez as Montoya was alright, if a bit wooden. Cain was undoubtedly the worst. Her lines were bad, and the actress couldn't sell them at all. Everything she said was confrontational teenage cliches and she spit them out in a really forced way.
I found that even the action weird. Every scene was just them beating up on goons. There wasn't much flow to the fights, it would just shift from one slow-motion set-piece to another. It was also weird that Black Mask himself didn't seem to be much of a threat either, and his gang of masked henchmen kind of came out of nowhere.
I didn't expect to like this film, but I didn't want to hate it. I don't really ever want to see it again and I don't think you can reasonably say any of the supporting characters were given enough that they could carry a sequel. And I wouldn't want them to, without basically being revamped and giving proper character backgrounds, at which point I'd say get actresses who were a better fit.