I think that as long as you don't allow yourself to be blinded by the hype, you can learn something of use from any trainer, be it a positive training tool or a lesson in what NOT to do. PP, like other programs has some good bits, but it seems to cause such a rabid following that it turns a lot of people off. The trick is to keep open eyes and take only what is useful, and leave the rest. Telling the two apart is the hard part.
In some cases I think your best bet is to learn the horses' body language, and then watch THEM when these various trainers are working with them. You can see when techniques are really working the way they are proclaimed, or whether the horse is being nitpicked or frightened into submission.
"Keep a leg on each side and your mind in the middle"