Hi Francis. Sorry, I've never used it. I just picked it up for my upcoming JMT hike this Sept. But based on what I can see...it's a lot more intuitive than some other guides I've picked up (e.g. the one by Elizabeth Wenk). I also got the Harrison maps, but I will probably be selling those now that I've picked up Erik's guide. The topography lines are easier to interpret (print quality) on the Harrison Maps, but Erik's maps are more clearly marked, with more points of interest/features (e.g. I can easily see switch backs, mileage between points, camps) If I was really worried about navigation (earlier months, with snow over the trail) I would stick with the harrison maps for their broader scope, since eriks maps, while the same scale, are divided into smaller pages. Another plus for Erik's guidebook is his readily accessible elevation profile charts. I would signup for a free preview of his guidebook (he emails you a couple of pages from it) to see if it works for you. To me, its the difference between owning a Mac and PC--his guidebook is just extremely user friendly.
But keep in mind, if you're an extreme gram counter, the harrison maps would be lighter since you can mail the relevant portions to your food drop. Also, the harrison ones are waterproof/tearproof but Erik's guide is not.