According to a study in the August issue of American Journal of Family Practice, there is little evidence of benefits of colon cleansing (whether by hydrotherapy, or by ingesting strong laxatives). What researchers did find, however, was a whole range of side-effects, including abdominal pain, nausea, dehydration, electrolyte imbalance (minerals in the blood that help regulate, among other things, neurological function) and kidney and liver failure.
Bonnie - I have always been dead-set against colon cleansers. The majority of them do more harm than good, especially by eradicating your gut ecology and depleting essential minerals. While I have been a bit more tolerant of colon hydrotherapy (colonics), I only recommend getting them once or twice per year, maximum. However, I always warn that there can be side effects and injuries from doing them.