I've had two different types of prep, both using polyethylene glycol. Once it was with a gallon jug of the stuff and the other with just a half gallon plus some laxatives. There are other preps that only use over-the-counter stuff.
Because your stomach acid is going to be washed out too, there's going to be anal irritation. Using disposable wipes instead of toilet paper helps, as does putting a little vaseline on after each time on the potty. I found that the worst problem was that when I also urinated, the urine would wash down towards my anus and then it would REALLY hurt. If I leaned forward so the urine went straight down and no dribbling in the wrong place it hardly hurt at all. I wondered if there is some urea/acid reaction that takes place.
On the whole, I think the gallon jug thing did the better job, though after about a couple quarts it becomes a real willpower struggle to drink the rest.
I didn't have any output until I'd drunk more than half, more than two hours into the ordeal. Apparently this is fine. I remember having a lot more pain the previous time, which was with the half and half prep.
I had a problem with energy. It's hard to get enough calories on a clear liquid diet. My partner suggested that I try Gatorade Recover, which has protein in addition to electrolytes and carbs. Unfortunately, I thought it tasted more vile than the polyethylene glycol! So I didn't have much of it. But he found it useful. I also kind of wondered if just because it was clear in the bottle, it actually counted. I mean, raw egg whites are clear too, but I bet the curdle right up in your stomach.
My answer to the problem of just having sugar-water for the whole day is to make strained vegetable broth. Commercial vegetable broths all have pulp and are cloudy, not clear. Unfortunately, while a nice change from sugar-water, vegetable broth does not contain much in the way of calories. Chicken broth doesn't have much either, though it would have protein. You'd have to make your own here too, I think. I counted calories and I ended up having about 700 for the day. Having a mouthful of veggie broth after a glass of glop during the prep time also helped cleanse the palate.
Then, of course, the day of the procedure you're not supposed to have anything at all. Combined with the previous day's starvation rations, I ended up being very cold all morning, with no energy, and not much ability to concentrate. I had had the fantasy that I'd work for a couple of hours before going to the hospital, but ended up spending the morning huddled in bed. I didn't exactly feel sick, just cold. I also had a caffeine-withdrawal headache, because I didn't feel like having any the previous day, but should have, to stave off the headache which I traditionally get the second day I'm off caffeine. I brought a snack with me to the hospital: a Mountain Dew, a banana, some crackers, and a granola bar. That, combined with the muffin and juice they offered me in the recovery room, fixed me right up.
Both times I asked to have light sedation so I could watch the procedure on TV. The second time I had less sedation than the first. Next time I might see about no sedation at all. The first time I didn't remember much, just a little bit of them using the tiny power-washer to clean off places where stuff had adhered despite the prep. I remember more from the second time. It seemed to me that the colon walls were cleaner, which is why I think the gallon jug does a better job. There was some initial pain as they inserted the camera, which I don't recall with more sedation. So maybe I won't go with zero next time. We'll see. After the initial pain, though, it didn't really hurt much anymore as they pushed it up through the colon. And it was kind of cool to see the end where it attaches to the small intestine.
Afterwards, of course you have no bowel movements for a couple of days while you fill back up. (Yes, I weighed myself at the low point. It's about 3 pounds.) I found it interesting that for the next week or so my bowel movements felt much smoother than usual. I wondered if the clean-out process is good for you all by itself. But then everything got back to normal, so maybe it's irrelevant.
I also participated in a research study trying to see if they can get the same data about your likelihood of having colon cancer from less invasive tests such as blood test and stool samples. Apparently a lot of people don't get colonoscopies. Which I can sort of see, it being a pretty annoying test. For the stool sample, they wanted my entire poop and gave me a little bucket to insert into the toilet seat to collect it. (It was a lot!) Then I got to send it next-day air to the study. Lots of opportunities for jokes: "I gotta call UPS about shipping this shit."