Simic has hit the nail on the head with regards to what UP is all about.
It's not a parenting guide. It's not a "Your child has defied you. March him to the naughty step and say these words..." book. Nor is does it tell you to allow your children to misbehave and then talk about it afterwards.
I don't think anyone is a "full paid up member" because it just lays the foundations for a different way of thinking.
It's a philosophy based upon how what we instinctively say, is not necessarily the best thing to say.
It's helped me to stop (just for second in some instances) and really think about how I want to respond to my DD. Doesn't even need to be anything major. Just the difference between snapping 'Don't be so silly' and actually trying to understand why she's in a little ball of rage.
As others have said, it's a slow burning thing. All about respect, understanding and honesty.
That's not to say children whose parents haven't read the book are going to grow up angry and browbeaten.
My parents didn't read the book, and I always felt unconditionally loved.
But I feel, by putting into practice some of the things I read in the book, my family will be more peaceful. My child(ren) will grow up respecting everyone equally, no matter what their age. My child(ren) will grow up with a handle on their emotions, and will understand how their actions effect other people.
None of these aims are exclusive to "followers" of the book. Nor are they only possible by reading the book.
But UP helped me understand who I want to be as a parent. As did other parenting books that I've found just as influential.
I think the trouble comes in with the term UP, in a similar way to GF (Gina Ford). You hear the intitals (and I'm as guilty as the next person) GF and think of strict routines and ridiculous rules that people follow blindly. But I bet there is actually a very small number of people who follow that book 100% to the letter. Most people will have read it, and then applied some of the ideas to their life.
But people use the term GF to quickly explain the sort of parent that they are, i.e that they are parent who has a relatively strict routine.
I use the term UP to define my parenting (only in certain circles, as most people haven't heard of it) as that instantly tells people that I don't do naughty steps/reward charts and I try and be as respectful and flexible as possible when it comes to my DD.
On the flipside, there are plenty of people who like routines that will never have read GF. And people who do the things that I do that have never read UP.