In general this is a terrible course, which was not aided by Jeon's teaching. The course, which attempts to combine applied and pure complex analysis, fails spectacularly and teaches students neither. The textbook focused on applied methods and the lectures focused on the theory behind these methods. This meant that the homework was entirely computational, and the theoretically motivated test questions resembled neither the homework or the lectures. The class average was between 30% and 40% on each exam. The exams had a few T/F questions and the wild guesses on those determined entirely whether or not I was above or below the class average. Jeon was hard to reach and didn't provide many outside resources. Furthermore his handwriting was illegible, which made the already obscure lectures harder to follow. The only good thing about this course was the generous curve. If you are interested in theoretical complex analysis, I would recommend Honors Complex, which is much more interesting and provides a comprehensive introduction to the subject. If you are interested in applied math or physics I would recommend taking a course in the applied math department.
Bogwang Jeon is an excellent professor. He doesn't only understand his material himself, he also genuinely cares about his students and the quality of his teaching. As another reviewer mentioned, if you didn't enjoy your previous Calc course, he will make you enjoy math again. His lectures follow the book closely enough that it's easy to follow along and read up on things, but are structured and presented differently enough that it's really worth going to class. In addition, he's also just a fun guy to talk to.
I took this class Spring 2015 with Professor Jeon and would recommend him at least for a silver nugget. He is the first mathematics professor I have encountered here that uses colored chalk, and it greatly helps in gaining a visual understanding of the material. He explains most steps very clearly, and if a student does not know how to get from one step to another, he'll answer the question right there. Sometimes his handwriting can be a bit unclear, but because he writes down most of the steps, it's easy to follow his work and point out any (rare) mistakes he makes on the chalkboard. He also takes the time to go through the proofs of various theorems, which I appreciated because most of them were unfamiliar to me before taking this class. He teaches well, and I think I learn better in my one hour in his lecture than in three hours with the textbook. At the end of the semester he spent the last couple of lectures on complex variables. He does have a Korean accent, but it is not thick enough to hurt his teaching. He makes an effort to learn the names of his students (and even recognized me outside of class and said hello), and cracks jokes every so often about his Korean-ness and about how mathematics is better than physics. His tests are fair, bordering on easy for those who are mathematically gifted. There are no trick questions - it is just six questions, where usually one of them is multiple choice. I am not particularly strong in mathematics, but I have done well under his methodical teaching style.
I took Calc III with Professor Jeon class last semester (Spring 2014) and was thoroughly impressed. He always started class with a review of the previous day's material, which probably helped the class members for whom math doesn't come easily. If you're strong in math and think this would be a bit tedious, it was. However, mathematics is all about developing complexity from basic rules, and his teaching style definitely helped me remember the basic rules we were using each day. This was especially helpful on the first class of the week, considering 4 days had passed between classes, sometimes without homework to keep the material fresh. Professor Jeon accommodated questions well and was patient during lectures. His English had a Korean accent, but it wasn't thick enough to obscure the material. I could see how a student might have trouble with his accent, however. It mainly interfered with his understanding math questions, which are often asked without being terribly organized or coherent beforehand. However, he allowed other students to chime in if they thought they could help class move along. He would always use many colors of chalk to help communicate ideas to students, particularly when graphing equations. He was also funny. He made fun of his Korean-ness by asking a student to teach him British-accented English so he would sound smarter. He made a joke about Gangnam Style. He pretended to be a North Korean spy, which I can neither confirm nor deny. If a student would try to start some banter with him mid-lecture, he would usually play along and have some fun before going back to work. The only reason I would not give Professor Jeon a Gold Nugget is that he did not teach me how to think creatively in mathematics to prove new theorems. It is very hard to teach this, but if I were not initially mathematically inclined, I doubt taking his class would truly inspire me to love mathematics or teach me to think like a mathematician. All in all, though, I was very satisfied with my Calculus III experience. Professor Jeon was funny, clear, and diligent. His minor shortcomings should be no problem to anyone with any self-motivation to do well in mathematics, and would not detract from the grade of anyone less mathematically inclined. 9/10
Professor Jeon is honestly the best. He made me enjoy Calc again. Don't get me wrong - Calc III is hard. It takes effort to do well on the tests. However, going to lecture with Professor Jeon is the best. He always starts the day with a joke - and keeps making the class laugh in between his examples. He's a great dude, and he teaches well. He never uses notes in class - his whole lecture comes from him talking through definitions, examples, and drawings on the board (It's really cool and scary - he knows so much about math he can actually RECITE the textbook. I love it, because that means he already knows the information, so he always focuses on how the students are reacting instead of how he should be recopying his notes), but always goes at a great pace and always asks for questions. What's even more surprising is that if you ask a question he will actually take the time to answer you the best he can, and will be really receptive to staying after class if you have something on your mind. I would recommend supplementing his lectures with the book and his office hours if you don't understand something, and sending him an email at his math.columbia.edu account always gets a response within the day. All in all, he's a great professor, who knows what he's doing and is able to communicate it to his students. He is worth taking, and a great guy to get to know.