I love NH groundwork and foundation riding skills. Works wonders for establishing a respectful, trusting relationship with the horse, no matter what "brand."
The best advice I can give you on what to look for and what to steer clear of is to think critically. There are good methods in almost every "main" version of NH, but because of the publicity and the marketing that goes into mainline NH there can be a lot of fluff out there.
Read Tom Dorrance and his philosophy, read some equine psych books (Cherry Hill's How to Think Like A Horse
is pretty good), and get a baseline idea of the underlying philosophy and herd dynamics, so you have something to run the potential fluff through, and decide whether or not whatever a given trainer is telling you would be beneficial. Also, you know your horses, and have a better idea of how they will react to certain exercises than any DVD does.
I personally like Clinton Anderson, as far as having a complete outline of the training process, but as far as what I actually put to use in the arena, I borrow from almost every name out there. I really can't think of any that I would specifically steer clear of.