You need to gauge the competency of the teachers in YOUR area. Some places have awful "teachers" that do not help and, in the worst cases, hurt your score. Other times, the teacher can really motivate you to study and help you achieve your target score. Ask other pre-meds in your area if they've taken a course and sit in on a lecture if possible.
If you don't have good teachers, then the class may still be worth it. You could easily form a group with some of the students which will help in studying and provide camaraderie.
However, the main reason to take a prep course is the sheer amount of practice material they give you. Note that I didn't say content review material. This is because there are usually better options than Kaplan or TPR for the different subjects. Still, if you need practice material and only practice material, sign up for an online course and save a little money.
The last aspect of this decision is whether you need to be paying for a class to study more. If it helps, then do it.
As an aside, never pay attention to the guaranteed score increase because they are typically gimmicks. In other words, they give you an abnormally hard test your first day which is designed for the majority of students to do badly.