Full disclosure: I'm also a retired ICL instructor, having taught from 2000-2016. Also, caveat -- the Institute changed hands at that time, and I haven't seen the updates they've made in the last almost-2.5 years.
Honest opinion: When I started teaching, and long before that, ICL was the best course of its kind out there. The curriculum was wonderful and the one-on-one instruction couldn't be beat. However, we know the internet has truly exploded on the scene since that time, and now there is such an abundance of offerings. You have much more choice, and what you choose will depend a lot on your goals and your learning style.
I think ICL is especially valuable for those who:
-- Are primarily interested in the magazine market, or in starting out by writing short pieces
-- Want one-on-one instruction
-- Want an online class as opposed to in-person/classroom setting
-- Want to set their own schedule -- ICL offers great flexibility for you to take the time you need to complete the course, even if life gets in the way
-- Want the six upper-division undergrad college credits, which can apply for teacher re-certification as well as a for a bachelor's degree
-- Want to earn those credits for a fee that includes books and is much more affordable per credit than most offerings
-- Have an interest in writing educational NF for schools and libraries, as many instructors have expertise with this
Not every applicant is accepted, no, but the reason it seems most are is because they would also be accepted into this college-level introductory course if they were registering for it on a brick-and-mortar campus.
I hope this helps!