I just took the FOI 2 weeks ago. Overall, I felt like the CFI course was super similar to every other KING course I taken. It felt like the same videos as well. I've used the King's courses for Private, IFR and COMM. I think that they're great at presenting the concepts and fundamentals of aviation.
My only minor gripe is that there was a difference between how the practice questions were presented in the study questions versus how they were presented on the exam. But that's the way life goes. For the FIA and AGI exams, I'm also studying using Sheppard Air.
Good Luck!