After his first quarter of teaching he had good reviews; overall he was sitting at around 3.8 or a 4+ or something in that range. *He looked good*. Hence why so many people eagerly signed up to this class. It takes a lot to drop from such high reviews to such low reviews.

He angered a lot of students. Not just some of them. Practically everyone in Spring 2019 had a bad experience in his class. I could go on and on about this, but I don't need to. Just read these other reviews if you want an idea of this monstrosity.

I took this class in the online format in Spring of 2020. I learned a lot in this class, Professor Rosario makes the content interesting and is very helpful during office hours and on CCLE. On CCLE, he will give extremely clear answers which was really helpful especially due to the online format of the course. During office hours, he is approachable, and I was able to get clarification on some of the harder homework problems.
In addition, towards the end, he covers some topics that are not required by the curriculum but are very interesting and useful. He also gives advice on his experiences from industry etc in the last class which was awesome to hear especially since he is a UCLA alum.
Exams are challenging, and reflect lecture and homework content. The homeworks were reasonable although they usually had a couple of difficult questions that usually took a little more time.
I highly recommend Rosario for CS143!

From what I can tell from previous reviews, RRR is definitely a changed man. Lectures were occasionally amusing with some jokes here and there. However, lectures are also like a firehose of information with some lectures having over 80 slides. However again, lectures are recorded and I found going through the recordings to be really helpful (had to spend like 3-4 hours for a 2 hour lecture though :/). Homework was alright, but ofc it's always better to start early, but he allows 2 late days for an assignment (6 late days total) and since there are only like 7 assignments and 5 that are seriously graded the late days are actually pretty generous. Tests are hard, but they were open note so take good notes and index them or get an iPad and it should be alright.

I can't speak for previous quarters, but for spring online remote instruction he was one of my most considerate professors when it came to students needs. There were a couple of homework assignments (no projects) but none were particularly time consuming and he was quick to clarify and answer questions. It also seems he downsized the number of slides from previous quarters, so referencing the slides/handouts made the questions pretty direct. I felt that attending lecture (or watching lecture recordings) was still essential as he more clearly explains and annotates the slides in class. The final was a whole lot longer than the midterm but he went over what topics would appear on it so I didn't find any questions to be unfair.

Despite expecting a bad experience from all the previous reviews, I think Rosario took a lot of the student complaints from previous quarters and made a bunch of improvements and it shows. Overall, I feel like I learned a lot and was able to enjoy his class without having to stress about my grade.

This professor definitely has changed from the previous quarters, I'm not feeling anything negative mentioned in the previous reviews in his class.

This professor was great this quarter. I came into this class just hoping to learn SQL, but ended up learning a lot about a lot of different databases in a good amount of detail.
He isn't condescending or rude in the slightest, patiently answers everyones questions (though he does ask people to see him after class if the class is being held up too much) and is honestly kind of funny.
The projects were also well designed and extremely reasonable. Overall, I definitely recommend this class.

Now that the final lecture is done, I just want to tell everyone that Rosario is amazing and you really really shouldn't be afraid of the old reviews.

Just a warning this is the professor you are taking this class with:
https://www.reddit.com/r/ucla/comments/1ci3cl8/cs144_campuswire/

The class this quarter seemed to be a significant improvement over last year's. Prof Rosario removed all midterms/finals and replaced them with quizzes every 2 weeks on Respondus, which you can take any time within a day or so.

Lectures were mostly good, especially if this stuff interests you but sometimes sped through difficult concepts. For example, he spent maybe a grand total of 15 minutes going through some of the Recommendation Systems Algorithms and quizzed us expecting to know every little detail about its implementation.

The projects were a bit disorganized, likely due to him developing them concurrently with the quarter. Despite this, the first project was beneficial, as it made me create a front-end webpage for job searching using pure HTML, CSS, and JS. The second project, where we had to implement mongoDB, graphQL, and tRPC APIs was decent but definitely could've been split into multiple projects as some of the API endpoints didn't end up being used in the frontend.
The third project was all about WASM, but during the WASM lecture, I thought he went a bit too quick -- we had to write some code in Rust, which wasn't covered at all.
The fourth project was cancelled and he gave us all 100%'s, but he said it would have had something to do with Kubernetes, which is something the final project required anyway.

As for the quizzes, they were mostly fine, since you could refer to your notes and the slides, but some questions could require you to dig a bit further...

The final project (which is similar to your CS35L project but with extra features such as deployment, CI/CD, PWAs, caching, and authentication), was pretty straightforward, but our group started 2 days before it was due and treated it like a hackathon -- we somehow got 100% though despite not implementing all the features. The final exam period was dedicated to group project presentations, with individual or two-person groups exempt from presenting. We also provided peer feedback during these demos for credit.

Honestly I do not know why Prof Rosario has such bad reviews. I will say that at times his lectures were a bit unclear (specifically a few topics like B+ trees) and he uses very specific phrasing/wording sometimes that you must know or else you could lose credit but his slides are helpful/easy to read. His class is recorded and you can reference the slides as well. For a CS class, I felt the homework was light and helped you prepare for exams. The coding projects were quite enjoyable in my opinion and not overwhelming. The exams were extremely challenging (the first one was okay but the grading was a bit brutal (i.e. very little partial credit) and the second one was probably the hardest exam I have ever taken and was super long), but the curve was so generous that it didn't matter. I performed below average on both exams (and failed both), did well on homework, and ended up with an A- so the curving must have been very generous. Rosario has even mentioned that you would have to try to fail his class, so at the very least if you put forth effort, you will pass the class. I think you learn very useful/applicable knowledge in this class and the lectures were fairly interesting to me and he would add in a joke here and there which was nice. I didn't do the readings and he doesn't require attendance. He is also friendly if you go to his office hours. I would take this class again!

Very very helpful class and material. CS143 + CS144 pretty much covers the basics for full stack software engineering. The biggest pro of Rosario's classes is that it is packed with information, so you can learn a lot from him. He is able to convey his ideas clearly, but I think one potential problem is that some examples in his slides could cause confusion for a beginner, but he might not be aware of it (as someone who already understands the knowledge). This requires a little more effort from you to understand and do some minor research, but I would say it's definitely worth it considering the amount of knowledge you will gain.
Regarding previous comments, I agree that Rosario has a problematic personality. In particular, his biggest problem is his pride / ego; apart from that, he appears to be mostly rational. Nevertheless, I don't see why this should affect the rating of his course. His primary job as a professor is to pass on the knowledge, and the primary job of me as a student is to receive this knowledge; and I think he has done a well-above-average job in doing so. In terms of personality, Rosario has his own journey that he should work on, so is everyone. I don't see why we should be concerned for him.