If you would have posted your question three months ago, my answer would have been totally different. I spent Summer 2018 with a stack of materials that came highly recommended by my big sib, Nymeria (famous for the SDN 100-day study plan), Zendabi, and KoalaT. In retrospect, I am 100% certain this was the ideal plan for my self studying, which resulted in a 516 (that unfortunately had a 122 in CARS). I applied and got one only one interview (late in the cycle) and am sitting in limbo. So I had to repeat the MCAT and took it this past Saturday.
I didn't decide to repeat the MCAT until a very depressing New Year's eve when I had still yet to receive an interview. Crunched for time, I decided to take a course. I know from all sorts of feedback that online courses are not helpful and you might as well save the money and study on your own, watch videos as needed, and hire a tutor for your weak areas. So I bit the bullet and took a live MCAT class in a classroom setting. It cost me $1700 and I had to drive there and back three days a week for nine weeks. This was by far the best choice I could have made and I wish I would have taken a class the first time. Again, had you asked me a few months ago I would not have said this.
What a course did for me more than anything else was make my time efficient. Rather than reading and watching videos, I was able to come to class, learn the content and reasoning, and then ask questions as needed. I was in a class of twelve people who were surprisingly nice and supportive. It comes down to how you connect with the teachers and all of mine were really good. What made my experience particularly great was that the author of the chemistry books was my actual teacher. So maybe my situation is so unique that is not representative of what you are considering. But looking back at my two experiences studying for the MCAT, and I will say both were great, taking a live class with amazing teachers is by far the very best way to prepare for this exam. Just my opinion and others may differ, but I know the other people in my class would agree 100%.