It will certainly clean you out. God only knows why anyone would want to willingly take it, though. It is NASTY!!! We're talking makes-you-sick-to-your-stomach nasty. Imagine the worst-tasting cough syrup you've ever had to take and imagine drinking a gallon of it and you'll have some idea of what it's like. Most people dread drinking it worse than they dread having diarrhea all day, much less the actual colonoscopy procedure. And more than one person has vomited it back up it's made them so nauseated.
A number of people on here have said that the Fleet's phospho soda doesn't taste quite so bad and there's a whole lot less of it to drink. Also, there are pills that you can get that work as a laxative. And in Canada there's a product called Pico Salax that has gotten rave reviews--it's supposed to taste like Tang.
But I think you'll find most everyone on here pooh-poohs colon cleansing anyways. The people who have tried it have only gotten relief for a few days before going right back to their old IBS symptoms. Of course, for people with chronic constipation, a few days relief can be a lifesaver, especially if they have a special event coming up or trip they want to take. But unless you are constipated, I really wouldn't recommend you do a colon cleansing; there's really no point. Contrary to some of the claims of the websites selling stuff, poop doesn't get caked on and built-up in your colon. The only time you have poop lodged in there is when you are constipated. Other than that, it's flowing through there naturally at its normal rate of speed. The smooth muscles of the intestines undulate like a snake--and how often do you see a dirty snake? It is wiggling around too much for something to stay stuck.
Those of us who are chronically cleaned out by bouts of diarrhea will certainly tell you that having a clean, empty colon sure hasn't done us any good; we'd happily spend a little time backed up.