I had a failed one, although I was meticulous about following the instructions. I switched GI doctors, and my new one had me do a prep in pill form. It worked perfectly I think it's called SuPrep, and it's quite expensive, but with insurance and my doctor's pharmacy's discount, it came to $50., which I felt was well worth it. It consists of two sets of 12 little pills in two sets, taken six hours apart. The instructions are quite clear, and the tiny pills are easy to take. The only problem is that it's hard to get a full night's sleep because you have to take the second set in the middle of the night and then, if you're lucky, you have to wake up very early for the procedure. It wasn't a big issue, since I was out for the procedure and rested once I got home. My doctor told me I was really cleaned out for the procedure.
On a lighter note: Paraphrasing Woody Allen's description of a colonoscopy - - -
Death is like a colonoscopy. You're lying there peacefully, floating along, not caring about what's going on around you.
Life, however, is like the prep!