Disappointing.
Hard to follow the narrative. Not as well developed a story (or just poor screenwriting and/or editing). Unnecessary moments, such as the 'musical' segments.
Violence, foul language, breaking the fourth wall; do these traits sound familiar? It should: Birds of Prey felt like DC were imitating MARVEL's Deadpool. A very surprising departure from all of DC's previous films.
One unique feature of superhero/villain/comic-book films is easter eggs. There's hardly any in this film. References to the wider shared universe, teases etc make these comic book films fun, but there's near to none this time.
And don't wait around for the so-called post credits 'scene', not worth it. Read on below in the spoiler section for a little more detail on this.
* SPOILER CONTENT *
Barely any prior explanation/inference leading up to Black Canary's canary cry; only comic book fans would have expected and understood that. Some prior explanation re metahumans would have helped. For the majority of casual audience members, the scene where Dinah Lance uses her canary cry would have been a 'WTH' moment: "Wait, what?! She has super powers?!". That specific part of the scene was so out of place, so random, it's as if the scene was shoehorned in.
Hats off to the VFX though; the power of Black Canary's special ability is felt and photographed beautifully.
The end-credits so-called 'scene' is pointless. It's not an actual scene, but simply an audio clip. The screen stays blank after the credits finish, and Harley Quinn speaks to the audience, about to spill a secret about Batman. But the audio sequence ends before she finishes her sentence.
* END OF SPOILERS *
WHAT THE FILM DID WELL:
. Dark moments worked well; it's DC's forte
. Fight scenes
. Humour
. Special effects
. Villain was menacing