I gave it a grudging 6/10.
The cast is good, and the story occasionally clicks.
But conceptually, its a mess, and it wastes a bunch of time and characters due to confusion about what it is. And I am mildly pissed about wasting Cassandra Cain (who i care about far more than nay of the other characters) in such an abysmally mundane role.
If this was really the result of two competing scripts, one BoP and one Sirens, than they both must have shared the same basic conflict outline, and either the Sirens script was really bad, or they were worried about Ivy and Catwoman stealing Harley's thunder.
Black Mask, a stolen diamond, the moral ambivalence of the tale, and its madcap energy... all that screams far more Sirens than Birds. This feels like two movies: a fairly decent Harley Quinn solo movie and a half-baked and sometimes non-committal Birds of Prey script. Montoya and Huntress are largely good, but that's because Montoya gets to be a straightforwards police antagonist for much of the movie, and Huntress is a mixed blessing, since her late entry into the story proper as an actual character means we see too little of her, but she gets to be accurately portrayed as well.
Black Canary feels like she's suffering from adaptational choices determined by a desperation to fit her into an awkward role that doesn't really suit her. The actress is actually very good, but the whole attempt to make her a wannabe-amoral associate of Black Mask ends up feeling like a waste of time.
Cassandra Cain is an insult to the idea of adaptation, let alone the character.
When it's trying to be a Harley movie, it's fine. When it tries to be a Birds of Prey movie, it honestly comes off as promising in potential but very weak in execution. Like, I find Huntress more interesting than Harley, particularly once Black Canary gets to act like Black Canary around her and they have chemistry, but its Harley's film first and foremost, and I just don't think that jived with the Birds of Prey as a concept that well.