Is A De-tox A Good Idea?

De toxifying the body of all the nasties it has accummulated over a period of time can be a good idea, but it needs to be done carefully. Some de-tox diets advocate drinking only water and very strong fruit juice for a few days, as well as taking herbal supplemts to get the body to rid itself of toxins.
This is great if you are young and healthy, but if you are 40 or over, you may need to talk to a herbalist or dietician before embarking on such a regime.
Fruit juice and diuretics can put a lot of pressure on the kidneys and so may not always be beneficial if you have any kind of kidney trouble or have diabetes or any blood sugar imbalances.
Some people find that even de-toxing for one day a month can have benefits, but again this depends on your state of health and inner organs.

Hedgehog gave a great answer.  I would add that medical doctors do not adhere to the theory that we have all this undigested gunk in our intestines.  Drinking  a lot of water,eating a lot of roughage and taking a fiber supplement like Metamucil will keep your digestive track healthy. Also EXERCISE, EXERCISE AND EXERCISE will keep things moving along.