I was originally intimidated by the harsh comments here about Prof. Hempel, but they are not true. (Which I think is partly because she’s also changed since first several quarters here?)
Prof. Hempel gives engaging lectures, though we didn’t get to finish the last part due to UCLA canceling classes. But her notes are neatly arranged and provide you full guidance in the process. She is a ‘magician’ who demonstrates complicated theorems and proofs in a straightforward, clear and sometimes novel way.
She’s also very helpful through the course, and you can tell she has so much passion about the subject.
The homework grading policy has changed since this quarter: if you get 50% more on the homework, she will give you full credit for the homework, which accounts for 10% of the course in total.
The two midterms are, to be honest, a bit hurry for majority of the people, having an average around 70%. But it’s bearable and somehow trivial if you do the homework diligently and have a clear understanding of the subject.
I can’t speak of the final because it’s take home this quarter due to the corona virus situation, but it’s by no means hard given the time limit is not enforced and you have access to everything you want.
Above all, this is my personal experience. If you are considering taking 115A, Prof. Hempel is a good teacher who can lead you through. But I definitely recommend auditing different lectures before you make the decision. Experiences vary for different people.

Professor Hempel is a great teacher, you can really see how she loves mathematics and how excited she is about teaching. She posts her printed lecture notes online so you don't need the book. She also gives study guides before the exams which are a very good reflection of what is going to be on the actual exams. During her office hours she was always very helpful, ready to answer all of your questions.

I found Hempel to be a really good professor! Lecture is clear and engaging, and she's always very open to questions and invested in student learning. She provides lecture notes for the class and follows them to a tee, so if you like that style she's definitely for you. Exams are easy if you have mastered the lecture and homework, but given on a time crunch so be prepared to write fast! Overall, I think Nadja did a great job of introducing us to proofs and covering the material for the course, and I hope to have her again as a professor in the future.

Actually I've known her "reputation" since I first came to UCLA, and thanks to that I could get a spot in Math 115A with a late second pass.
But those comments are too harsh, she's at least not bad.
Her lectures are mostly clear and sometimes confusing, there are some examples along with some concepts. And if you loss your track you would just look at the lecture notes after. The lecture note basically tells everything that she would talk in lecture, so preview and review would benefit you a lot.
Homework is easy and straightforward since all of the hard problems are not graded. And according to the syllabus, as long as you get 50% percent, you would receive full mark (it's really easy to get 10 out of 20)
Midterms are 50 minutes and the majority of the class would run out of time. The median is about 25/35. But overall the questions are not hard, even though you could not finish the exam, you could at least make sure that what you've done is correct.
Due to the coronavirus, all final in math department is 24hr (essentially no time limit). The final is not easy and there are one or two hard problems, which worth about 15% of the final. However, there is no time limit so take your time to figure it out.
Generally speaking, she's not bad and it's definitely OK to take her math 115A.

Honestly, I think Hempel gets a bad rap. Going in, was pretty concerned about her poor reviews, but she's honestly very invested in making sure that everybody in the class understands the material. Compared to my other math profs (lower div & upper div), I think she's on the clearer side of things, and is more than willing to take time out of class to answer questions/make sure that you fully understand everything in the class. She's also super helpful in office hours! She also has a set of really helpful typed-up notes that match exactly what she talks about in class, plus some extra comments - super helpful stuff!

There's weekly homework + midterms and a final, but I didn't think that they're especially bad for an upper div math class; certainly better than all the other one's I've taken since this one. It's a jump up from lower-div math classes in general, so you'll definitely want to watch out for that.

Overall, pretty solid class that gave me a good understanding of linear algebra. Hempel is a super nice prof and a solid lecturer. Would definitely recommend!

Professor Hempel is really nice as she truly cares about her students' well being. The quizzes and midterms are pretty easy, and even so, she still grades leniently. There are totally six quizzes that constitute 50% of the grade and she lets us drop two of the lowest scores. The midterms are 10% each, which actually makes each quiz (12.5%) more important than each of the midterms. The only shortcoming of this class is the extremely lengthy final exam which took forever to finish. Overall, she's a great professor who knows the materials well and I would definitely recommend taking 115A with her.

One of the few math professors that explain stuff clearly in lectures so that you don't need to resort to the textbook.

I would definitely recommend Hempel for this class. She has obviously taught this class many times and she explains concepts and proofs really clearly, and this is coming from someone who usually can’t follow along in class. The tests were fair and the curve was generous and she obviously cares about her students.

Will recommend taking 115A with Prof. Hempel. She has taught 115A six times and I think she did quite a good job explaining the concept. She's also very nice and helpful. The workload is light, she gives you full credit on homework if you did 50% of the problems right. The exams are not hard, although the 24hr final was a bit time consuming. Overall it's a good experience.

I definitely think some of the comments here are too harsh, but I wouldn't go so far as to say Hempel was an outstanding professor. Both midterm averages were around 70% with no curve. The grading scheme was meant to help you if you did badly on a midterm, as one could be dropped, but then the final is 65% of your grade so if you do badly on that you're basically screwed. She does get all her notes from another professor, so if you ever need clarification you can read the other version.
I think my poor performance in this class was a mix of my personal lack of understanding and ability to grasp conceptual topics as well as her sometimes confusing lecture style where she basically just copies her notes onto the board.
Overall if you have to take it with Hempel, it's not the end of the world, but if there are other professors available then take theirs.