If you're asking 'does it work' in the modern medicine proven-by-double-blind-clinical-studies sense, the answer would be a definite no. But then sometimes you do get some people swearing by it.
Myself, I'd call myself an agnostic on this topic. I don't buy into the whole energy lines mumbo jumbo but it apparently the body does respond (at least locally) to having pins shoved into it and twiddled in some ways that aren't completely understood, so it may be good for something. It's biggest problem is that no-one has yet developed a truly convincing placebo (it's a bit difficult to pretend you're sticking a needle into someone when you're not) and as such, hasn't had the sort of development that you would normally get through doing blind trials and stuff.
It's really important to consider the trends here. Until recently we haven't had credible placebos for acupuncture. Now, two main placebos are being used:
1. `Stage dagger' needles, which touch the skin but don't penetrate because the needle retracts into the handle. These have been tested to see if people can tell the difference, with quite convincing results.
2. Needling outside the meridians.
Neither technique is ideal, and neither can be double-blinded, only single-blinded. But the point is that as the placebo control improves, the effect size decreases. Non-treatment controls in the past showed big effects for acupuncture, but sham controls show effect sizes that fall below statistical significance. If we ever do develop a fully credible double-blind technique for acupuncture, I would predict that all effects will disappear.
I should mention the imaging studies that show effects on certain brain areas in response to needling. I'm not a neurologist, but I suspect that there is little or no consistency between the studies. It really isn't surprising that needling triggers a response in the brain. The question is, is it therapeutically relevant?
Overall, my position has shifted over the last year or so. Before that, I was fairly confident that acupuncture was effective for some types of pain and for post-op nausea and vomiting (PNV). I am now much less convinced regarding pain, as it can increasingly be explained by placebo effect. For PNV, it's well known that pressure on the wrist will help, and anaesthetists commonly do this. It's a curious neurological effect but it's not acupuncture.