JSL is no joke. I studied it for 3 years at university. After my second year, I got a monbusho scholarship to attend Saga University with home stay for the summer. There were 23 other Americans, and my spoken Japanese was second best. The best was a guy at Cornell University, who was clearly smarter than me - he also studied JSL (the lady who wrote it is at Cornell). When I went back to school for 3rd year Japanese and the third book, I dominated the class.
JSL is still used at top schools in the US for serious learners. The best school in Missouri, Washington University, uses the system. It’s a fast way (the best?) to learn to speak, learn accent, build listening comprehension and learn to use grammar. The structural pattern sections in each chapter are gold. I used to sit on my couch in college and read them over and over again. Definitely best used in a classroom setting, but anyone could benefit from the explanations and drills with audio.
The romaji is funky. No one writes it like that in real life, but it was easy to adjust after moving to Japan. It’s true that my reading and writing was slower than other comparable students, but being able to communicate with my host family and actually make friends at that time was far more important.
Edit: My Japanese teacher at university was a weird little dude from rural Japan, and one of the best teachers I’ve ever had. The teacher matters.
Edit: For people who’s interest in Japanese is to learn to read manga, for example, JSL is not the way to go. For people who’s primary motivation is to communicate with people effectively, it’s a serious system.