If you understand the method then go for it. Parelli himself says it's not for everyone because not everyone can comprehend how a horse's mind works, nor do some have a big enough scope of imagination which plays a key role in NH.
I've studied Parelli and really enjoy it and like the way they do things.
As others have suggested hiring an actual trainer, that's always a good idea but in the long run it will cost more. First off, you'd have to find a trainer that does the things you're intersested in and that's not always easy. Second, though you would get one on one experience, it only lasts for as many lessons you have. The good thing about a DVD or book is that you can always go back and look/read if you get stuck. You dont have to keep forking out money to get someone else's knowledge.
If you have the money, perhaps do both. The Parelli Level 1/2 DVD's are very helpful as he breaks it down, step by step as well as working with a horse who's new to the program so you can see how he fixes certain problems as they arise.
They can be pricey, but they're no more expensive and in some cases cheaper than any of the other NH trainers. And though many people don't like him for HIM, i can't see how they can bash his methods and results. They work, so long (as said before) as you understand how and when to use them.
I've been working with my mare's using the NH method for 8 months now (i studied it a bit before in the past but never buckled down to trying it until recently) and believe me, it works. They are so much more respectful, responsive and interested in their work now.
Sure the Parelli's have marketed basic knowledge but who cares so long as it works and helps you and your horse? Just ignore the mustache and you'll be fine lol