I am 44 years old, and I've had Ulcerative Colitis since I was 23, so I've had 6 colonoscopies so far (have to have them every 2 years go forward). Like it was said earlier, the actual procedure is nothing. You may be able to stay awake during the procedure (I still remember watching the monitor on my first one), but more than likely you'll not be aware of it and not remember a thing afterwards.
The worst part by far is the prep the day before. The best way to describe the taste is a mixture of raw eggs and snot. Like someone said, get a lemon or 3 - it will help deaden some of the taste. I usually stop eating a full 2 days before, so the bathroom trips aren't as many since I usually am unable to down the whole gallon.
Bottom line - don't fret about it. While the prep sucks, the procedure is easy, and it has literally saved my life. Every man should have one once they hit 45, and then every 5 years after if they are healthy.