Chiropractic seems to work well for short-term fixes for back pain; long-term most back pain gets better on its own and clinical trials show it pretty much doesn't matter what you do (chiropractor, physio, exercise, nothing).
As a few others have alluded, the medical community looks down on them in part because of the beliefs of a lot of chiropractors (from their training) that pretty much all health problems are caused by "subluxations" in the spine and their attempts to treat things other than back pain. The anti-vaccination position of chiropractor schools and professional organizations doesn't help, nor does their doing things like suing a writer for libel for saying they can't treat asthma by manipulating your back. Then there's the whole issue of neck manipulation possibly being linked to strokes.
There are tonnes of articles about the dark side of chiropractic on sites like
www.quackwatch.com (and its affiliate
www.chirobase.org) and sciencebasedmedicine.org; but I personally don't see a huge issue with using it for back pain if it works.