Acupuncture is a complex "alternative" modality because something physical is actually happening - thin needles are being stuck through the skin and manipulated. So it is therefore not impossible that a physiological response is happening. It is much easier to comment on things like homeopathy and therapeutic touch where literally nothing physical is happening and the plausibility for any benefit is therefore zero. So if I try to answer the question in my title, much explanation and qualifications are required. To answer this question - does acupuncture work? - my current best answer based upon available evidence is a qualified no. This answer is not changed by the most recent study of acupuncture that is being touted by the press as evidence that acupuncture works. (Here is the original study, but a subscription is required.)
Let's first look at this study, which was a German study of acupuncture for back pain. Dr. Heinz Endres studied 1,100 randomized patients with three treatment arms. The first received standard therapy - massage, anti-inflammatories, and heating pads. The second received acupuncture, and the third received sham acupuncture where the needles were inserted but not deeply, and not manipulated, and not in traditional acupuncture points. The study found 47% improvement in the acupuncture group, 44% in the sham acupuncture, and 27% in the standard therapy group after 6 months.
This single study, even taken just by itself, falls far short of demonstrating that acupuncture works. And of course we have to place it in the context of plausibility and the entire acupuncture literature. We also have to identify appropriate sub-questions.
First let us consider the difference between "real" acupuncture and "sham" acupuncture. Acupuncture is based upon the ancient and superstitious pre-scientific notion that there are lines of mysterious life energy (chi) flowing through our bodies, and that the flow of this energy is responsible for health and illness. Acupuncture is supposed to free up blockages in the flow of chi energy. I grant this idea a scientific plausibility of zero - meaning we can safely discard it.
What does the evidence show for the chi theory of acupuncture? The evidence is overwhelmingly negative, and this study supports this negative consensus. Most well-designed studies that compare traditional and sham acupuncture show no difference between the groups. In this study the two groups were 47% and 44% respectively. This means that it does not matter where you put the needles or if you manipulate them in any way - that's because there are no lines of flowing chi.