Colon cancer takes a decade to develop from polyp to tumour. The main purpose of a colonoscopy is to find and snip any polyps there could be in the colon. Then you'd be safe from colon cancer for a decade or more.
You would normally not feel much: the sensory nerves in the colon are only really able to feel if you have gas. And many hospitals offer sedation, so you wouldn't need to experience the procedure anyway. The worst part is the restricted diet for a week before (avoiding foods that would cling to the intestine wall) and emptying the bowels the day before. If you know that you have healthy kidneys, do ask your doctor for the good laxative that is a small dose that doesn't taste bad.
Myself I have an unusual genetic condition, so I unlike normal people, I do a colonoscopy every year. I stay awake and watch the monitor, because I actually want to see what I look like inside. At the end when you reach the small intestine, the cilia on the walls inside there are quite fascinating.