I have experience Ki...in fact I am experiencing it right now.
It is very hard to describe key since in is very, very conceptual and sematical in nature.
Is a nutshell it is the life force energy that emanates through every living thing. Everybody experiences it in some form or another otherwise you wouldn't be alive or have conscious thought.
The ability to harmonize with it and "walk the fine line" between the Yin and Yang of it is very difficult to do. Something that I have had trouble doing and being able to replicate on a regular basis.
Is it metaphysical, no I don't think so. Ki is conceptual in nature, but it is real.
Some describe it as a light, as water flowing, or a sensation of heat. Can't say I have experienced that type of phenomena....but I will tell you that I have experienced it in many, many ways! Being alive is a Ki experience in itself.