I used Suprep with my first colonoscopy this past fall. I don't have anything to compare it to, but I think it was fine. The taste sure wasn't great, but it was manageable enough. I drank mine once at night and then once in the morning I think? Or maybe it was twice in the afternoon/evening? With three kids to take care of, my brain gets a little foggy on the details!
But anyhow, I thought it was fine. I never gagged or felt like it would come back up. It was, however, very noticeable when it began to work. I was in the bathroom at least every 10 minutes for a 2 hours or so and then it would die down. By the end of the second dose, the liquid coming out during a bathroom trip was clear, so I assume it did its job.
So, although I've not had a TON of experience with different kinds of prep, that's my 2 cents.