If you would have asked me six months ago about courses, I would have said (and probably did say) that they were a waste. I studied on my own using the official SDN study plan with TBR for sciences, TPR for P/S, and EK for CARS. I studied for about five months, took the exam over a year and half ago and I got a 516, which would have been fine if not for my abysmal CARS score.
On a whim, and TBH overcome by a spike of emotion on New Years at having no interviews, I decided to repeat the MCAT. I chose to take a class this time around, despite my reservations. Hands down, taking a class was by far the best decision I could have made. I paid $1700 for my class, which was five or six lectures each week and office hours almost every day. The teachers would stay until every last question got answered.
The thing about taking a class is that it saves an enormous amount of time. My first time studying, I watched way too many videos to get one small concept cleared up and 95% of what I watched was stuff I already knew. I wasted so much time. This time, I kept a running list of questions and had them answered during office hours. No wasted time watching things I didn't need. No having to watch multiple videos until I found what I needed.
I still have my reservations about online classes, but I can say that as far as in person classes go, I'm a complete believer. I got a 519 this time around after studying for a little over two months. I studied half as much as my first time, and did better. One caveat, one of the teachers is the same guy who writes the chem books. He seriously is a god when it comes to chemistry and physics. If you don't get him it won't be the same.
Classes are worth the money if you use them wisely. When I add up everything I spent the first time studying and compare it to what I spent this time, it's about the same. A place like SDN or r/mcat will shame you for considering a class (see the comments), but if you think one will help you then do it.