Is it always useful to spend money on an MCAT prep-course? I have heard people say that it was not worth the money.
Would it be better to just buy an MCAT study guide?
This is a great question and a loaded question. The short answer is, "no, it's not always useful to spend money on an MCAT prep-course." But the reality is that it's a question where the answer depends completely on the person.
On the pro side, a class with a couple good teachers and a couple great teachers can motivate you to study, give you shortcuts so you don't have to spend as much time reading, give you insights into the exam, and give you a resource to get your questions answered without searching several websites. The right course will give you a supportive environment surrounded by people going through the same things as you. It can make studying easier and more time-efficient.
On the con side, a class may spend too much time on a topic you know well and thereby waste your study time. With limited hours in class, they may gloss a subject you don't know that well. A class with a subpar teacher (or teachers as may be the case with a given course) can be disocuraging and you'll be studying on your own anyway. If the teacher simply reads from the book to you, then it would be an absolute waste of time.
Without knowing the exact course, exact teachers, and what the student needs, there's no way to answer your question besides a generic answer. A few things to keep in mind is that even with the classes that offer over a hundred classroom hours with a live instructor, you're going to be spending about three times that many hours outside of class studying on your own. So even if you take a class, you really are studying on your own. With a course that offers only fifty-something classroom hours with a live teacher it's an even bigger outside-to-inside ratio.
If you can study on your own, then that should be your first choice. You can buy all of the prep materials you need for around $300 to $500. You can buy full-length CBTs from AAMC and other sources for between $200 and $400. If you really need help on a specific subject, a really good tutor might run $50/hour. If you need ten to twenty hours of tutoring it'll cost about $500 to $1000. So you could get one-on-one specialized learning and all of the materials you need for somewhere between $1000 to $1900. It'll take more work on your part to organize your preparation schedule, but you can customize it and save money.
I don't want to downplay the utility of a course for some people, as they offer everything that person needs. But if you are asking "do I need a course?", then you are probably one of the people who doesn't need one.