He is a very very nice guy but his lectures lacked in helpfulness and clarity. He often ran out of time and was speeding through examples and would skip steps that he just assumed everyone knew how to do. He also often used proofs to explain concepts which is just kinda useless in this class and examples would have been more helpful. I don't think I ever walked out of the class understanding what he was really teaching that day and had to teach everything to myself. His homework was also significantly longer than any other of the 32B classes and were often questions not in the book so they were harder to complete. The first midterm was significantly easier than the second and the final but the final was also 24 hours and open note (coronavirus) so it took a while but you could eventually get the answer. Other than that he really seemed like a nice guy trying to help his students. I would say he's much clearer in terms of answering questions through email rather than if you were to go into office hours (he often gets back the same day and is very specific).

Lectures were a mixed bag. Sometimes they were super unclear and I didn't understand anything. Other times they were fine. And many times Sangchul would zoom through examples because he was running out of time, often skipping crucial steps assuming we knew what he did. Often he would show some proof of some theorem that did not really help the class. Overall, lectures were fine (except for a few).
But despite that he really cared about students. Sangchul created practice tests for every midterm and final, and also posted solutions for them (and for both real midterms too). This I really appreciated about him and these practice tests often mirrored the actual exam. And he often lightened the mood in lectures by making some (pretty bad) jokes and drawing smiley faces and the occasional "aha!"
It is Sangchul's first time teaching so I do give him some slack. Overall, it wasn't that bad. Tests are very computation heavy as opposed to proof/theory heavy. But being computation heavy does not mean easy. Homework can be a little long, but overall not that bad. The textbook is not required because he types up the homework and posts it.
So would I take this class again with Sangchul? I would say yes.. despite all the negatives. He is not a bad professor. At the beginning it was a bit rough, but as time went on I felt he got a lot better at teaching the class. It's tough but you will be fine with Sangchul.

He is cute and nice. Take his course if you can

Did a weird thing during my quarter. He pre recorded the lecture that had subject content but still did a live lecture going over examples. Very helpful since he was basically doing double the work and putting in double the time. Unfortunately his grading was very slow because of it.

Lee did a flipped course format which meant you had to watch a 1 hour 30 minute pre-lecture before each 50 minute lecture. This was pretty bad because it took up a lot of time. Because of this I ended up not attending discussion since the TA just did simple examples there. The textbook was the most helpful. Lee's examples in lecture were fairly good. I wouldn't take this class with Lee just because of the flipped course format.

Given the circumstances, Professor Lee did a great job. We had clear pre-recorded lectures, weekly hw, and a couple of quizzes. (10 I believe). Professor Lee was nice, respectful, and helpful in office hours. He even had extra office-hours on weekends before the exams. The pre-recorded lectures were delivered only a couple hours before the actual class, but he always posted them. He even finished a week early. I would recommend Professor Lee. Given all the work and effort he put into this class, he deserves much more appreciation.

Note: exams are a bit difficult, but doable.