Reiki started out as a healing system. There is much debate going on among the Reiki-ists as to how it started, who is doing it right, what is or is not Reiki... and who can get the legal rights to which version of the best, most true, or most effective style. I have really no interest in the debate, other than for overall entertainment value -- which is not much. Take my views with the requisite salt lick.
From my perspective, Reiki is a sort of access filter to what can be very loosely described as "Universal Life Energy". As a filter, it has its own quirks, its own rules, and thus has reasonably coherent ways that it can be utilized by the magickal practitioner. Reiki is a healing modality.
The Reiki Attunement method is very simple. The person's aura is opened up, the requisite Reiki symbols are placed inside, with at least on full set placed into the base of the skull. The symbols may be placed in other areas as well, such as the third eye, and the hands. Once the symbols have been placed into the energy system, they tend to activate rather quickly, beginning to clear the energy channels of any blocks between the physical form and the spirit template. Each level tends to hit different areas of the person's life..Reiki I may stick to the physical, while II may hit more emotional issues, etc.
The question is, how does it heal?
From my experience, Reiki is at least moderately sentient... Actually, it may be more accurate to say that it is a hard-wired system more than a sentience per se. It seems to be able to find it's way to what needs to be healed, but the question is, how does it know what to do? My working theory is that it somehow links up with the "spirit template" of the what it is to heal, and works to bring the physical form closer to that design. This is the only reasonable explanation I can come up with, as I have seen it work to a reasonably good degree on physical objects other than living beings.
Which brings us to the point of it's use to a magickal practitioner. Reiki is not just useful for those new-age "good think it into health" sorts. Yes, it is a nice tool to have around if you get hurt... it can help speed physical recovery up a bit... but is good for so much more. If you are into designing servitor thoughtforms, for example, you could add in a Reiki subroutine into the design, and have it maintain the servitor to keep it in line with the intent it was created, and have less chance of the said creation going rogue on you.
I have found Reiki to be a very effective tool WHEN YOU WORK INSIDE ITS PARAMETERS. It will not work in harming others. For example, if you were trying to re-write someone's energetic form, and this conflicted with the template, the Reiki will fight to erase the other working, effectively nullifying the forced change. If you are trying to aggressively harm someone, Reiki will not help. Now, if you are defending yourself, Reiki is a great weapon... Surrounding someone with a Reiki Cage, for example, is amazingly effective. It takes advantage of any issues the attacker might have, and neatly defuses any malicious attacks. I have found firing "Reiki Viral Fleschettes" down an attack path to be a really nice deterrent to most bothersome people. They follow the persons' need to attack right to the source, and works on helping the person heal their issues that have led them to bother you in the first place.
Of course, this pre-supposes the person attacking is in need of healing, or is in the wrong. If they are not, the technique can backfire. Reiki works by pulling the healing energy through you, so you do not use personal reserves. Of course, the thing to look at is the "through you" part.