Warning: my answer might be out of focus for you because it takes into account only the information on your question, while some of your comments complement that information.
Unfortunately, some instructor do waste students' time by doing a very poor job; I'll assume this is the case here (but note that sometimes students feel that a course is done very badly but realize later that they learned more than they expected: student need not know better than instructors what they really need).
Here is basically what I told to students complaining that their instructor made the course impossible to follow when I was responsible for a 500 students, 20 instructors course (they were not the first to complain about these instructors, and I had other evidence preventing me to blame them for the problem): I know your teacher is not the teacher you would have wanted, but I cannot do anything beside reporting the issue to him, which will certainly not solve the problem instantly. We have the same issue as teachers, having some students that often do not know the prerequisites for the course, are not willing to learn them as we tell, and/or do not believe us when we say that the course needs a fair amount of personal work. Both teachers and students should do their best in the situation they are given, which means that you can and should try to talk with your teacher to mitigate the issues you have, but at the same time be prepared for the issue not to be solved before the end of term, and find a way to learn what you have to learn. There are books, there are other students with whom to work, etc. If you think it is best, don't waste time with the instructor's course and learn by yourself while keeping in touch with the important information (homework, exam dates, etc.)
So to state this again in fewer words, it seems much more efficient for you to make your best to master the course content, judging this content from the syllabus and what related books discuss, rather than expecting next year's instructor to be better.
I would say that the expected outcomes of such an attitude should be:
you learn valuable things and pass the exam, because ultimately it tested what you learned,
you learn valuable things and pass the exam, because your institution realizes that there is an issue and makes it right somehow,
you learn valuable things and fail the exam, which turns out to be indeed impossible.
Even in the last case, you get more from this attitude than dropping the course altogether.
Edit: this answer was written under the assumption that the class is mandatory, as I (mis-?)understood the question. Of course, for a class that is not needed or that can be switched with another, then these would be options to consider seriously. If the course can be delayed without losing the benefit of the other courses, this would also be an option to consider. So my answer is really about what to do if dropping or failing the course implies one has to take the whole year again.