We have talked about Effortless Mastery here a bunch of times.
I've read the book about 3 times.
Yes, it totally liberated me. I did indeed have to tailor some of it's philosophies to rock playing as opposed to jazz, but yes it was extremely helpful.
A few thoughts-
It's not a philosophy for beginners in my opinion. One has to practice and stay on a regular playing regimen in my opinion. One can't gloss over the basics and expect to be a "master musician" that is misleading. I do believe the more advanced player can choose their avenues of study. I also believe that you don't need a whole lot of technique and training to still make amazing music.
Kenny Werner is a gifted player, most of the rest of us are not. Talent is a big part of making music.
Totally letting go and playing from a meditative state is ok at times but personally I have to keep looking at my band members, thinking a bit about changes in the music, upcoming fill ideas, tempos, groove and how my body/posture is reacting with the drums.
So I believe in my case the meditative exercises can be used as a concentration practice tool; that is what I personally really need. I can get distracted and forget about what I'm there for. But sometimes getting distracted is fun, after all I'm a rock musician

So it's a great read all and all and I'm still reaping it's benefits.