I'm only in Book 3 and my perspective is one of an advanced jazz pianist who is taking the course. I should also note that in the beginning of the book the author warns of "getting into futile debates" with those who aren't familiar with the material.
First of all, the course is NOT about "tonal harmony" or serialism, nor is it a gimmick. The course covers counterpoint in an original way as well, but I'm not there yet.
The cost of studying the course at PCC should be well under $2k.
I've heard EIS described as the "unified field theory of music", which I think is fitting. It doesn't necessarily contradict of traditional theory, but it defintely takes you out of the "iii-vi-ii-V7-I" tonal box immeidately and in a very logical fashion. You could use the techniques to write things that relate to the normal major/minor diatonic universe, but that's a passing consideration in the overall course. It's an original/alternate way of looking at the machine of music all the way up to the way-out 20th century techniques.
Book 2, which seems to be the 'germ' of the course, is about voice-leading. You learn to write music 'one line at a time' using a basic technique which he calls "change of position". This basic concept is explored to its limits by the end of Book 2 so that you can basically voice-lead any chord to any other chord. BTW, the method doesn't limit you to one way of voice-leading. Parallel movement is OK as well as "none of the above" voice-leading, but you have to indentify it as such.
"Equal Interval" refers to the bass notes (or "root tones"). The bulk of the assignments are a series of controlled experiments writing voice-led progressions of a single chord type over the various Equal Interval bass line movements (half-steps, whole steps, minor thirds, major thirds, perfect fourths and split-octave) both ascending and descending. Various techniques for altering the individual voices are explored along the way. I guess you could think of it as an expanded version of figured bass.
From a writing perspective, it continues to open many fresh harmonic doors that I knew little or nothing about. I've even picked up some chord types/voicings that I never thought of using before, which is no small feat for an advanced player like me. The neat thing is that it's not just the usual case of "here's a wierd chord, play it in parallel fashion through the cycle of fifths", you actually get to understand how to use those structures, resolve them and voice-lead them.
As far as in improvisational concept, that has been a little slower on the uptake for me. 99% of my jazz playing has been centered on the iii-vi-ii-V7-I/major key/minor key/blues/standards universe. The closed position structures used in the book (3,5,7,9; 5,7,9,11; 7,9,11,13; etc,) are basically rootless structures, but they're not used in the way you'd typically play on "Autumn Leaves", so my normal muscle memory is useless in this context. This dog's gotta learn some new tricks with the left hand. However, once it's comfortable I'll be doing things that'll make Fred Hersch drool!
The system also uses several unusual scales, again, not part of my vocabulary yet.
This is a link to a homework assignment I wrote at the end of Book 2:
http://www.audiorecordingandservices.com/Page_100.mp3 The solo is a throw-away and doesn't have anything to do with the course per se.
If you want to hear some music done by some graduates of the course you can go here:
http://www.vi-control.net/forum/viewtopic.php?t=961 Peace,
J