So, for me, the biggest grip I had with SS was the lack of cohesion and focus. It had such a weird pace, with a crazy and perhaps unnecessary amount of flashbacks, most heavily misplaced, breaking even the momentum at some points. And yet, even with all the flashbacks, all the character felt half-baked to me. It was like the movie was constantly telling you, “I want you to care for this character, then I want you to care for this other character and now I need you to relate to this one!”. So on and so forth. Like, whose movie is this? The movie wants you to care for most of these characters, but who should it belong to? Why should this be important to the story? I thought it was confusing and mismanaged. The writing certainly didn’t help. It’s a basic story and a bad script.
It bothered me that an iconic character such as the Joker didn’t get the big entrance he deserved, because he kind of appears out of nowhere, and stays on the sidelines for most of the movie. His scenes are short, and not memorable enough to have an impact on you. You don’t get to see a fully developed character, and it felt like that constrained Jared’s performance, or at least the way we perceive it.
I love Margot and I love what I’ve seen of Harley Quinn the character, both in Assault on Arkham and in the Batman games, so I enjoyed Margot’s performance but not straight up loved. She certainly looked the part and had some brilliant moments, but she was limited by the script. I felt like Margot’s Harley was 80% there, if that makes sense. She was the star of the movie though, at least to me. You know what was the best moment of the film, imo?
The one where she is on the top of a car, alone and sad, thinking that the Joker is dead, and that she will never get to be with him again. But as soon as the rest of the squad meets her there, she lifts her arm, composes herself and becomes Harley Quinn, the crazy goofball again. I appreciate moments like those, and I wish we had more of them. That was very well-done and gave her some depth.
Viola Davis was great as always, and say what you will about Will Smith, but the guy oozes charisma every time he’s on screen. It is partially his movie, too. All the other characters… not interesting enough, despite the movie trying to convince you otherwise.
Tl;dr - I would give it a 5 (maybe 6) out of 10. Maybe I’m being too harsh on what is was meant to be merely a comic-book movie. I just feel like the potential was there and so I’m disappointed that an amazing concept like this was wasted. It had all the ingredients to be a critical success, but in the end, it felt rushed and underdeveloped.
And going off the ending, there better be. 
I'm really curious tho, because
needed more scenes like that
I know he's a minor character but damn, poor sis