I consider myself proficient with the UI of Pixinsight while admittedly not proficient with all of the processes of PI. I have Warren Keller's book and have learned from many of the great tutorials online over the years however I just have not been able to take my processing to the next level. I keep reading about how Adam Block's tutorials are excellent however it's really expensive if you want to get both the Fundamentals and the Horizons sets. ($450 total).
There are several topics in Fundamentals that I'm just not interested in learning more about, such as pre-processing, BPP etc...
I decided after weeks of debating that I would give Horizons a try. Included are complete processing examples for several datasets. I figured that with my familiarity of using PI for almost four years that I would not be lost by jumping into the advanced lessons. I've so far watched through one of the processing examples and beginning to digest the actual tool lessons within the Horizons set.
I'm sure that I would get plenty out of the Fundamentals, but for someone with my experience with PI jumping into Horizons is perfectly fine. I'm amazed so far at the little conveniences I have picked up. Like quick keys and ways to compare images as well as a better understanding of the "why" in what we are doing. I'm really impressed so far.