I spent 6 months studying pressure points in a semi-private class. The instructor was certified through Dillman, but his background was TKD followed by years of Qigong, Chin-na, and acupuncture/acupressure healing.
My personal take on it was that a lot of it is hooey. However, there were a couple important things I learned from it:
1) Some of it really does work. Not all of it, not all the time, and not on everybody, BUT some of it DOES work, and sometimes very effectively. I even did a couple KOs back then, and only one "victim" was also a student. My training partner was known for knocking out skeptics on our floor in college (only the willing ones, of course). :)
2) When attacking any part of the body, consider the organism as a whole. When you watch a KO in any sport (boxing, Muay Thai, Savate, MMA, etc) look at what caused it. Was it a single blow? Was it a series of blows?
A good example of the "PP KO" is Pedro Rizzo vs Tra Telligman. Rizzo throws the leg kick, and Tra visibly winces and his body reacts to the kick. Rizzo follows it with a right to the jaw that puts Tra down. If you watch his eyes in the replay, they glass over and he looks GONE until his butt hits the mat, which wakes him up.
This is the same effect that the "PP KOs" achieve. They hit multiple areas of the body to overload the nervous system and cause a shutdown. Was Rizzo/Telligman a PP KO, or just a good right to the jaw? Nobody can really say, I suppose. The PP folks will claim that it was PPs, others will dispute it. I only mention it because the effect is real, if the practice isn't.
Do they work without sparring or alive training? No. Many PP practitioners can only do them when their partner is standing still, waiting for it. However, the effect that they are trying to discuss and employ is not ALL bunk.
When we trained, our instructor made sure we learned both the healing side and the hurting side of PP study. His massages were AWESOME. I also learned a lot of acupressure remedies that worked very well for an assortment of things, like back pain or headaches.
The bottom line: it's not something that I would ever try to depend on, but sometimes PPs lend an ancillary benefit to what you are already doing (BJJ, Savate, etc)
~Chris