1 star for Rosaline, 1 star for Dario, and 1 star for Rosaline's father. Now onto the actual review!
This movie...what can I say about it? So, I love a good retelling of stories, especially filling in gaps that were present in the original, or telling the perspective of a character we don't know much about, but this movie...well, it only succeeds 50% of the time. After the first half of the movie when Rosaline gets over Romeo and the movie goes full throttle with making fun of the original Romeo and Juliet, it's much more enjoyable. Therein lies the problem.
The character of Rosaline in the original Romeo and Juliet wasn't much of a character at all - she was a plot device for Romeo to be at the ball and meet Juliet. In the original, Romeo simply had an unrequited crush on Rosaline, and Rosaline was never in a relationship with him. In fact, she made a vow to shun men and become a nun. If writers actually delved into this aspect of her character, i.e. rejecting romance, there would be a great story. I think that what many writers who attempt to tell "Rosaline's" story don't understand is that she doesn't need romance. In every adaptation about Rosaline, she is in a relationship, whether with Count Paris, Benvolio Montague, etc. Which is a shame, because I think an adaptation with a Rosaline who rejects romance would be great.
The movie tries to address some of these aspects, like Rosaline is rebellious, likes horseback riding/sports, wants to go on adventures around the world, etc. Similar to her original character, she does not fit the mold of a conventional woman who marries/has kids. But instead of going all in with this, the movie gives her a romantic interest in the form of Dario. While I quite liked his character in the movie, it's still a shame that they needed to give her a love interest at all, when the original Rosaline wholly rejected romance and relationships.
More about the characters...while I praise the actors for Rosaline, Dario, and Rosaline's father, most of the characters in the movie are not as great. I don't like that they made Paris a stereotype of a gay man (and Rosaline's best friend aka the gay best friend trope) i.e. dresses flamboyantly, speaks flamboyantly, etc. Like previously mentioned, after Rosaline gets over Romeo, she is a much more enjoyable character. I don't like the way the movie pitted Rosaline against Juliet for Romeo in the first half.
And finally, the best parts of the movie are when it pokes fun at the original Romeo and Juliet. Even in Shakespeare's own time, there were many who considered Romeo and Juliet to not be his best work, and it isn't. I enjoyed the movie poking fun at Romeo and Juliet's "deaths", i.e. how flawed Juliet's plan was to fake her death, and then Romeo for actually dying (in the movie, this is subverted by Dario telling him Juliet is not actually dead). And the after credits scene, which shows that Romeo and Juliet are on a boat to Cyprus after faking their deaths, to "spend the rest of their lives together", shows us that their relationship probably won't last.