Mascal is a good teacher overall, but very difficult. He goes over everything you need to know in lecture, but that's the problem: you need to know EVERYTHING. It is very difficult to predict what types of questions he will give on exams, so you basically have to have a thorough understanding of all the material. If you are willing to really work, and want to learn OChem well, take Mascal. He is very passionate about the subject, but also very condescending towards his students.