I chose to use a prep course because I only had one shot at the GMAT before applying R1. I needed the rigor of a study schedule and accountability of classes (and money!) to force me to study.
My instructor was engaging and had been with MGMAT for awhile meaning he was excellent at addressing questions and leveraging different thought processes in the class to help us all learn. Example - I have a strong quant background (engineer) and he would call on me to answer a question when he knew I didn't have the right answer. This forced me to think through my process out-loud and learn where I went wrong.
Verbal prep helped me significantly with Reading Comprehension - my weakest area. I needed to speed up and the structure of how to read and answer questions was invaluable.
The online tools were probably the most helpful for me. Being able to track specific types of questions I was getting right and wrong along with time was key in my final week of self-guided study before the test.