Hey Lavos
Chi Kung rocks. Done correctly it can certainly be mind blowing, physically empowering, and spiritually grounding... all at the same time!
I studied with a master. My Sifu was an expert in many styles of internal arts and part of a few lineages that are quite old. The benefits of such lineages can not be overstated. When a Kung Fu master accepts you and becomes your Sifu (spiritual father), his Sifu becomes your spiritual grandfather, and so on. Thus you become part of a spiritual family of masters. The combined power of such things can be overwhelming... even if you discount the idea that many such lineages were started by taoist immortals or have ascended beings in their ranks.
I've read Mantak Chia, watched some of his videos, and find him to be decent. His being Thai, and not directly linked to one of the famous lineages often gets him some ridicule. I think this has more to do with people not liking his commercialization of the arts and sharing of secrets. Most of what he teaches jibes pretty well with what I learned.
Having said that, there are notable points where his teachings differ. He has different organ sounds than what I learned, for example. Not just different, but actually mismatched in places. Where he says one sound is for the liver, in my system the same sound is used for the pericardium. (I have to check that example, but the point holds)
Even if what Mantak teaches was perfect, and you could learn it just from a video, you would still want to seek out a master at some point. These arts run very deep. Many of them take decades to be considered a beginner. Having someone who has mastered them around you is beneficial in so many ways that I can not overstate it. Aside from correcting you, it is just an honor to be around someone who is demonstrably superhuman.
Ljosalfar is correct in his assertion that Tai Chi, Pa Kua & Hsing I are among the best of the bunch, though anything taught at your local gym or community center is likely to be heavily watered down. Water boxing, Mok Gar, and dozens of other Kung Fu styles also contain very advanced internal systems for Chi Kung, Nei Kung, Shen Kung and more.
Blessings. Namaste.
"Curiouser and curiouser..." ~ Alice
"Do not believe in anything simply because you have heard it. Do not believe in anything simply because it is spoken and rumored by many. Do not believe in anything simply because it is found written in your religious books. Do not believe in anything merely on the authority of your teachers and elders. Do not believe in traditions because they have been handed down for many generations. But after observation and analysis, when you find that anything agrees with reason and is conducive to the good and benefit of one and all, then accept it and live up to it." ~ Buddha