Being born and raised in the eastern world, surrounded by mysticism, I myself never been a skeptic of Ki. There are many interpretations of Ki. Some take it to the extreme mystical or supernatural, some take it to the physical aspects of it.
In Aikido we develop Ki from the outside towards the inside. We have to understand the physical side of Ki in order to understand the spiritual part of it. But, we will not be able to understand Ki if we are not able to accept the spiritual part of it.
My study of Ki is helped by training with Ki-no-Kenkyukai, although I am under Aikikai. It is quite regretful that Tohei Koichi-sensei had to leave Aikikai to build his Ki training methods for Shin-shin-toitsu Aikido/Do and Ki-no-Kenkyukai.
There is an element of physical form that let Ki flow naturally. That's why we learn many techniques when we first join Aikido. Then we are taught the mental aspect of it. Only when we understand these two aspects, the body and mind, we will be able to understand and accept the spiritual part.
The Aikido that my sensei taught now is quite different than the Aikido I remembered 5 years ago. I will not go into details, but it is now starting to resemble what Watanabe-shihan (Akuma-shihan) sometimes demonstrates. I'm quite amazed although not surprised how fast my sensei evolved. I've been with him since the first year he got his shodan, I witnessed him "grow" and "grew" with him. I do believe that this is because he is quite a spiritual person yet he is also a rational and logical thinker.
Well that's my story, anybody else? Perhaps the people at Honbu would like to enlighten us on Watanabe-shihan. Watanabe-shihan's uchi deshi perhaps, by some chance is a member of this forum?