I was part of the group last year, and wanted to chime in with just a quick update now that I have a book for every course I regularly play and more time with them. In short, I don't like playing courses that I don't have a book for anymore. They are that good. This year I started using a little bit of AimPoint on longer putts, and having the actual slope measurements almost feels like cheating. I'm 2 strokes better a round than I was at this time last year, into the + side for the first time in my life.
That's it really. Buy these books. They are fantastic.