Everyone learns differently, so what works for you will likely not be the same for someone else. If you have always used study guides to prepare for exams and it works for you, then by all means, this may be a good option for you.
But, to be quite frank, everything you could see in critical care nursing is fair game on the exam. So, if you are studying on your own, it can feel completely overwhelming to figure out how to concentrate your study efforts. That is why a CCRN review course is so valuable