It is hard to comment this course since your experience hinges on the amount of math you have learned. For those who are good at PDE and special function, Mueller will give you a lot of insights, and you will learn the connection between math and physics (For example, the spherical harmonic series Ylm stands for several electrical models, which are important in QM and classical field.) On the other hand, if you have not learned these things yet, it will be a pain in neck since Mueller won't go through details that he considers trivial, and you will need to read additional materials to improve your understanding. To make the matter worse, he does not have syllabus. However, Mueller is very nice and he is willing to explain everything if you go to his office hour or make appointment with him.
The lecture looks crazy and hard at the first place, but most of the content would not appear in the Midterm and Final (Mueller said he did not want it to be algebraic heavy). To be successful in this course, it is better to be familiar with multi-variable calculus (especially integral), some knowledge of PDE, and function expansion.