I think Reiki is a powerful and valuable spiritual and healing modality. While I am skeptical of claims that any energy "can only be used for the highest and greatest good," the spiritual force called upon by Reiki generally appears to have a powerful positive impact upon most recipients, with no negative side effects. Satayana has studied with teachers in three different lineages of Usui and Karuna Reiki and appears to have benefitted from their wisdom. I have seen impressive results from other Reiki workers, including a couple practitioners of variants like Otherkin (now Open Source) Reiki.
That being said, I've also seen some less-than-impressive things being done in the name of Reiki. One friend of mine had months of headaches, nausea and other issues after a masseuse helpfully manipulated his energy - thereby "fixing" his aura and screwing up various things he had acquired during his shamanic practice. (I should also note that he was never consulted about this impromptu psychic surgery: she simply decided it was part of her job along with the body manipulations). And I've seen plenty of people who get a couple of attunements, combine that with the stuff they read in Mantak Chia and the Tantra workshop they took last month, then set themselves up in business as "Energy Healers."
To give a nod to one of my favorite polytheists, Galina Krasskova, I blame part of this on Monotheism. It's part of the desire to reduce everything to one happy nebulous one-size-fits-all Truth. The kundalini serpent who travels upward and seeks to rejoin divinity is not the lightning flash by which the Divine transforms us in body and spirit. The pentecostal fervor of the Holy Ghost is not the healing balm that flows through the Reiki symbols: neither are to be confused with the furor Wotan sometimes brings to His disciples, or with the forces that generate body heat and muscle tension in the supplicant's physical body.
In this rush to create a generic one-size-fits-all school of energy work, these practitioners often miss this important distinction. Reiki calls upon something outside the practitioner: through the use of symbols this force is called upon for healing. This is something different than the energy which we generate in our capacity as an ongoing biological process. It's the old ceremonial magic distinction between invocation and evocation. In one operation you are calling in and acting as a channel, while the other involves the casting out and separation of spiritual force. It is easy to miss when you think the Divine only exists within your cranium: in theology as in all other disciplines, solipsism is an epistemological dead end.
The force that is called by and filtered through the Usui symbols appears to be exceedingly safe. There are no such guarantees with the force that is generated by a human body. When you get practitioners sticking themselves willy-nilly into various meridians, chakras and power points, it can become the New Age equivalent of a DIY appendectomy. People invariably steal the spectacular shiny points from a tradition while ignoring the dull, drab safeguards. And as Reiki (and energy work in general) grows increasingly popular, I see an ever-growing number of ineffective and outright dangerous "healers" giving a bad reputation to a perfectly good practice.