I've never had one, but after looking into it and talking to both my PCP and holistic health care provider, I chose to avoid them. This is simply MY OPINION and INFORMATION and of course there are other positions on this.
Here's what I learned:
The root cause of constipation needs to be determined so that your body can return to managing this on its own. Diets high in protein are sometimes lacking in fresh vegetables and fruits, sufficient whole grains, and very importantly, sufficient fats (good fats like olive oil). These deficiencies often result in constipation or irregularity.
Colonic cleansings certainly can help temporarily, but they also rob your body of important good bacteria in the colon. This can lead to future constipation and other colon problems.
Sometimes constipation starts in the upper intestines, and colonic cleansing only deals with a very small portion of the lower intestinal track.
Any time you rely heavily on "artificial" means of relieving constipation puts you at risk for your body not knowing how to take care of elimination entirely on its own.
So what would I do? Consider having one colonic cleansing to provide some immediate relief, but really evaluate your diet to make sure it is well balanced and has "smooth move" foods in it every day.
Again, just MY opinion.