Been there, done that. Dave Barry gets it right in this column.
"MoviPrep is a nuclear laxative. I don't want to be too graphic, here, but: Have you ever seen a space shuttle launch? This is pretty much the MoviPrep experience, with you as the shuttle. There are times when you wish the commode had a seat belt. You spend several hours pretty much confined to the bathroom, spurting violently. You eliminate everything. And then, when you figure you must be totally empty, you have to drink another liter of MoviPrep, at which point, as far as I can tell, your bowels travel into the future and start eliminating food that you have not even eaten yet."