I've found in the past that there's not much middle ground when it comes to how people feel about Parelli. They either think he's great or don't.
From what I've seen of his "product" (and as many on here have pointed out, he's selling a product) I'm not impressed. Not that he doesn't say somethings that are true, but if you are new to working with horses and your use of his product gets you a great, well trained horse then I'd say you were very lucky. That goes for anyone who is going to try and learn to train a horse via DVD or a book, not just Parelli's.
I've never doubted my extreme good luck at having come from a family of old horsemen. There was no such thing as "NH" when they were alive. They understood horses. How long it takes them to grow, how to get them use to new things, how to teach them not to fear and then, by default, be willing to do what you asked of them.
So I'll repeat what many have already said. If you're new to this, or even if you've done it, but feel you could do better, I'd suggest finding someone who can give you some first hand help. There was nothing like having my grandfather, great-cousins, etc... come over and tell me how to fix something and how to work patiently to teach my filly that she could trust me when I required something of her. I'm not going to say it's easy and it was certainly time consuming. Most of what I've seen of the NH products makes it seem like you can do it in shorter time than I've seen it take. You can't ask the DVD about something that came up. It can't give you pointers about, or explain, a behavior that wasn't put on it.
I think giving tips on NH is great, but it's not a product that you can sell. It's a service that you can provide personally, but unless horses have become standardized so that they all think, respond and act exactly the same (something we should all know is NEVER going to happen) then you cannot produce a product that will serve to teach someone via DVD or book how to train/teach a horse "naturally". You can teach horse psychology just like you can teach human psychology. A DVD or book will not cover the endless variations. If you don't have the experience (my grandfather always said there is no substitute for experience) then you're better off finding a person who does and get their assistance.