Parelli has many theories that are sound, and has developed many exercises that work. I think what turns many people off his methods is the marketing that's attached to them. There are a lot of other wonderful trainers out there who use the same or similar methods to Parelli, but they haven't stamped those methods with their own name--they simply call it natural horsemanship. IMHO, the best trainers are the ones who use methods/tips/tricks from a wide variety of trainers, and who never stop learning. My trainer has been riding and training horses for over 60 years, and she still attends clinics to learn more. You can never stop learning how to work with horses, and you can never find ONE right method that will work for every horse you'll ever own.
What I do like are Parelli's seven games. You can call them whatever you like, but the exercises do provide an excellent groundwork foundation.
My favorite trainers to watch and learn from are Jonathan Field and Stacy Westfall. Jonathan did spend time training under Parelli, and has adapted a lot of his methods, but is far more personable, and is arguably a better teacher than Pat. I find Stacy's exercises more methodic, but she explains them with such clarity, they're very easy to understand and try yourself.
DVDs are certainly not a replacement for in-person training, but can be an excellent supplementary tool as long as you're willing to look at videos from a variety of trainers so you don't become closed to using only one style. No one trainer's word is gospel.