I've had the opportunity recently (a couple in fact!) to experience reiki, from two different people.
The first one was provided for free at a place where I volunteered, along with a regular physical massage by someone else. Everyone went for the latter, and I was the only one to do the former... but only because the manager pulled me by the arm to try it.
I'm actually glad that he did... it was an experience for me at the very least. I was surprised at how similar it felt to what I would experience in my own sitting practice, without me actually putting in any effort. It felt very calm. It seemed like her "energy" was transferring through me. Though, towards the end the person started to feel unsteady to me, I think because she got distracted by someone talking to her.
The second time was someone who was visiting my friend from Japan. When I was getting ready to leave, she kept me down and then started to give me some kind of reiki massage. She seemed more sure compared to the other person, not shaky at all. The energy seemed more subdued, but it straightened my body right out... just with several different placements of her hands. This stayed with me for at least several days.
In the end, I think it's all just fabrications... but still a very nice one, nonetheless. Supposedly it was developed by a monk. I don't find that hard to believe since it seems similar to what I experience in samatha meditation, or even in the environment at a monastery that I go to.
It's very interesting, but still better to do your own practice, I think... and it's also probably not effective for a vipassana practice in itself.