I will preface this with saying I loved the books and have read them countless times over the years.
Now, I didn't hate this movie. It actually did a pretty (ha) decent job at staying close to the plot of the book, for the most part. There were some weird pacing issues and some scenes a little out of place, or rearranged a little? Some small details were missing that took away some weight, especially towards the beginning after Tally and Shay meet, and after Shay leaves.
Tally's inner monologue being entirely nonexistent also wipes some interesting character development from her, as we don't get to see or hear her being So thrown and disgusted by "uglies" that she's not used to seeing (including a character who i really liked in the Smoke but was removed from the movie, which also removed a couple scenes from the Smoke entirely as he was the central point of them).
⚠️ SPOILERS AHEAD ⚠️
Everything from when the Smoke got raided until the end was where it really deviated from the book and started breaking immersion for me a bit. Having the Entire scene cut short, with no attempts at running away, and everyone just being captured immediately really dumped the plot. Tally's betrayal being revealed through her jumping out and yelling at Cable for having Peris kill Az (which, btw, Peris being special at all was a weird change to make? and idk what they're gonna do about that if they make Pretties into a movie too) instead of her trying to run away, getting captured, getting recognized and freed, and then recaptured where Shay or Croy (I can't remember which) points out that she has 2 pairs of cuffs on, which the cuffs were supposed to be unbreakable except by Specials? just really.... idk. The whole ending just felt bad. They cut out a lot of the really interesting bits from the end and just... made it "cooler" I guess? The rescue from Special Circumstances was kinda lame and rushed too.
And on top of it all... Pretty Shay was just kind of scary! Why did they do that to her! And then on the way out they just.... kill off Peris for whatever reason. It's fine I guess because Special Peris was a weird choice.
Pretty Tally looked fine but I am disappointed they ended the movie with showing her as Pretty instead of ending on the line "Make me Pretty" like the book, but I do sort of understand why they would do that.
Overall, the movie would have gotten 4 stars if it weren't for the end. The deviations from the book made it just feel so weird and rushed, like they were suddenly in a hurry to end the movie and be done with it. The movie wasn't awful, but it wasn't the best either. A mid, 3 star watch.