What are the benefits / negatives from colon cleansing?

I want to do this but would like to know if there are benefits and problems with doing one. Any help with a particular brand that I should buy?

Well, Mike S was the closest… they do help you to poop, and they are often similar to a high fiber diet.
However, Colon cleansers are much more potent and powerful than a high-fiber diet, which is why they cost so much more, as he so eloquently put it. That's why many people prefer them if they don't like what would be required in a high-fiber diet…. it gets it over with faster. It's most beneficial for people that don't eat a lot of fiber, but want to change to a healthier diet with more fiber because it "gets you through the transition" faster. Simply put, in that situation, you'll have more liquefied stools for only a few days instead of a few weeks with a diet change.

Overall benefits for Colon Cleansers is that most people feel cleaner, healthier, and more energetic… like after a good poop or after you've been on a high-fiber diet for a while. Most people that stick with a full cleansing tend to repeat them every so often if they don't have enough fiber in their diet.
As far as brands to get, anything with a good variety of ingredients from a good quality company (which most that are sold in stores are) would be just fine. As long as there are at least 3-5 ingredients for cleansing, it should be fine, but the best bet is a few sources of fiber and preferably some probiotics (like Acidophilus, for example).

A lot of doctors may say Colon Cleansers are useless or a waste… but typically those are the doctors that haven't used them and haven't talked to people that have used them.