I've been running some flavor of Linux off and on since I was in college in the late 90s.
I decided to take the exam to become a Kali Linux Certified Professional as a next step into exploring a future in Information Security. I was already comfortable in Linux, comfortable with modern package management à la Debian and Ubuntu (Kali is based on Debian) and beginning to get comfortable with Kali specifics. I read the Kali Linux Revealed book and utilized the online course and quizzes to study for the exam. My first attempt was a bit disappointing as I just missed passing due to not having practiced the unattended installation exercises and remote administration via SaltStack. I recently retook the exam and am happy to report I passed! Now I guess I just wait for contact on receiving that expensive piece of paper. I know there has been a lot of debate about the relative value and/or merit of various certifications, but it was motivational so it was worth it to me.
Exam Preparation Advice
I think my number one piece of advice is to believe them when they say the online quizzes are not sufficient to prepare for the exam. While they are good practice in general, it mostly gives the "flavor" of the types of questions you'll encounter. I strongly urge you to not only read the book and the online materials, but do all the practical exercises you can. If you are already comfortable with Linux basics, and maybe if you've already spent a lot of time in advanced Debian package and repository work, then you can spend a little less time on those and more time on topics you're not as familiar with.
As I get even more deeply into the tools that are so easily installed for this distro, I'll blog more topics under the tag kali