The vast majority of treatments are unremarkable. Sometimes patients experience a slight tingling sensation as the needles are inserted, and in some styles of treatment there is a dull, aching sensation where the needles have been inserted and manipulated. This sensation is given the name 'deqi' (pronounced 'derchee') by the Chinese and is regarded as a sign that the treatment has 'taken.'
Since the use of plastic guide tubes with needles became the norm a decade ago, the effect of the slight pressure of the sterile tube on the skin surface has reduced the sensations associated with needle insertion considerably, and the increasing use of the Japanese-style sharply pointed needles has done the same. There will always be occasions when the more sensitive patient feels a little more discomfort, but most will feel very little.
Acupuncture needles are solid and extremely fine, as little as 0.18 mm in width, nothing at all like the large hollow needles which most people remember from their childhood injections or the nails which people see in cartoons about acupuncture! Ask your practitioner to show you how fine they are compared even to a household sewing needle, and you can be reassured that they are not going to cause much sensation.