Vaginal douches disturb the body's natural HP balance. It often irritates the vagina, but can actually do much more than that douches can flood your pelvic organs with bacteria and they increase your chance of infection. If you have ever felt a little sore and sensitive after a douche, don't be surprised.
Research found that women who douche regularly have a higher chance of developing bacterial vaginosis and Pelvic Inflammatory Disease. The latter can result in chronic pain and infertility. According to some, douches can increase your risk of HIV. What's for sure, though, is that douching after intercourse does nothing to prevent STDs.
Some research suggests that vaginal douches decrease a woman's fertility if she uses them around the time of ovulation. Women who douche regularly also remove the cervical mucus that naturally occurs. This mucus can give a lot of information about a woman's general health, and her fertility. By douching, you eliminate this benefit.
If you are douching to get rid of bad smells down there, you should see a doctor instead. Truly bad smells are usually caused by a disease, and it can be a sign you need treatment. The vagina cleans itself, naturally. It has done so throughout history, and it works perfectly fine. We have no reasons at all to buy expensive douches.
One other note never, ever have a vaginal douche before going to see your gynecologist/obstetrician. Nobody likes to be dirty and smelly during a doctor's visit, but your mucus is an important diagnostic tool.