IMO Kiatsu is a simple introduction to Japanese therapy that is interesting to Aikido people as it does utilize certain aspects of ‘Ki', albeit on a very basic level. I first read Tohei's pink book around 1984 as an Aikido Shodan ready to soak everything up. Although we were not a Ki Society dojo we used similar practices after training- as many martial arts dojos did. (My Japanese Karate teacher was a shiatsu practitioner and my Wing Chin Sifu had his own take based on Tui na. ) It was influential for me in that it led me to further exploration of Oriental Medicine and I later became a professional practitioner and teacher of a number of Therapies.
In hindsight and without wishing to sound dismissive, the Kiatsu book was simply a rehash of traditional folk therapy with some of Tohei's formative Ki principles added. If someone else had written it I doubt whether it would have been as popular. I never had the opportunity to experience Kiatsu from Tohei himself, (IHTBF?) so I do not know if there was more to it than is presented in his book or by his students. However, the people who I have felt who have ‘trained' in Kiatsu have never had anything special. They could easily have obtained the same or better skills by taking a short course in Shiatsu. (As an experienced professional therapist it is easy to pick those who have ‘it'- and those who don't. Now where have I heard that before?)
People who are interested in learning more about Ki work could consider Meridian Shiatsu, also known nowadays as Masunaga Shiatsu, or Zen Shiatsu in the USA, which works with Ki at a much deeper and more practical level. See Wiki link here for more info: http://en.wikipedia.org/wiki/Zen_Shiatsu
While I cannot say that Kiatsu itself had any effect on my Aikido I can say that it led to me practices that definitely did. Having ‘hands on' experience with patients 8-hours a day, 6 days a week teaches you a lot about the human body- particularly feeling energy, intent, center, Ki, Qi, however you like to describe it. For example: think kuzushi when turning the ‘dead weight' of an injured 110KG rugby player on a massage table built for lighter weights.
Of course, if you commit to any ‘hands on' practice with the same intensity you can develop a similar sensitivity- if you have a good teacher and focus.
I will add that there are many forms of Shiatsu some of which use Ki while others, such as the mainstream and more western Namikoshi method, do not. There are as many different styles of Shiatsu and related therapies such as Kiatsu as there are styles in Aikido and the discussions and arguments about the meaning of ‘ki' etc. are common in that community too! At least that is my experience. Think Yoshinkan=Namikoshi, Aikikai= Masunaga, Ki-Society: Reiki/Kiatsu.
Just my 2 Baht.