Difficulty of this class was in the teaching. If you dedicate enough time to learning on your own, it's fine. The material is honestly pretty useful for other STEM classes, but I did not appreciate it until after finishing the class.
Fun and interesting class, easy
I think this class is extremely teacher-dependent. I took it with Si Tang, and it was probably the second-hardest class I took in my four years at Duke. Be prepared to do a lot of calculus and to have no real idea what's going on, just arbitrarily trying different strategies hoping something works. It got to the point where I really didn't even retain any knowledge.
Mildly interesting, helpful for a lot of other classes
Really didn't like this class that much. The material was kind of easy at first but then it got hard fast. I couldn't really understand my professor and he went so fast that I was lost by the middle of the semester.
Having a good professor is important for this class. If you do all of the homework and attend all of the classes, an A is very manageable. Note that it is easy to get behind.
230 is an amazing class. One of the easiest classes at Duke (exams and homework are very easy). Most of the probability is pretty intuitive and the applications are not only interesting but can be very fun. Totally recommend!