What evidence is there for or against acupuncture being an effective treatment?
Acupuncture is based on the belief that diseases are caused by blockages of your qi, which flows in so-called meridians in your body. By sticking needles into those meridians you can manipulate the flow of qi and eliminate the blockages.
The concepts of qi and meridians are unscientific, they date back to a time where there was no scientific method and knowledge of human anatomy was practically nonexistant. There is no evidence that qi and meridians exist at all.
Sticking needles into your body still could have some effect, although it will have nothing to do with your qi. There are tons of clinical studies about acupuncture, but few conclusive results. A major problem is that controlling for the placebo effect is complicated, as people usually notice whether you stick needles into them or not. Some of the newer and better clinical trials used fake acupuncture needles that do not penetrate the skin. Another often used method is to stick the needles outside of the "proper" acupuncture points as a control.
Acupuncture is believed to be helpful in a large variety of conditions, I'll take pain relief as an example as it is somewhat plausible that sticking needles into your skin could have an effect on the perception of pain.
A review from 2009 in BJ concludes:
My conclusion is that acupuncture is just a particularly effective placebo.
Linde, Klaus, et al. "Acupuncture for tension-type headache." Cochrane Database Syst Rev 1.1 (2009)., based on eleven trials with 2317 participants (median 62, range 10 to 1265) that met the inclusion criteria:
You can find many more meta-reviews on http://onlinelibrary.wiley.com/cochranelibrary/search (search for "acupuncture"). E.g. here are more results (one paragraph per result):
It seems to be highly variable and condition-specific, although this latest review of reviews has some interesting results (mind you that's in paediatric population):
Efficacy and safety of acupuncture in children: An overview of systematic reviews. (highlighting is mine)