Reiki is absolutely wonderful and I'd recommend it to everyone. About twelve years ago I did the Reiki I and II course, mainly so I could use it on myself, and I've found it incredible. From time to time I've had treatments by others, but generally I simply use ten minute Reiki self-treatments as part of my daily heath routine, in much the same way that I'll go for a quick morning walk or do some yoga asanas. First thing in the morning, I find it a great way to wake up, fresh and invigorated, and if I'm feeling tired and run-down after a busy day, I'll do another quick treatment, just before I fall asleep. It's become a wonderful part of my daily routine.
In response to some of the previous comments, there is absolutely nothing weird or dodgy about Reiki. It simply working with the body's natural ability to heal and regenerate itself. In the East, Chi (as in Tai Chi) or Ki (as in Reiki), translates as the 'life force energy' (as Zen has mentioned), while in ancient India it was called Prana.
Although it originates in the east, it's totally non-sectarian, so it doesn't interfere with whatever spiritual one may follow.