I've had one. It's not pleasant, but not too bad. You can have sedation, but I didn't because I wouldn't have been able to drive to work the following day.
I had to have a special laxative the day before, and then only eat what was on the list they gave me at specific times, and drink quite a lot of water. That part was quite hard, but the laxative part itself wasn't too bad.
I felt quite queasy on the day, I think from the laxative. I was very nervous going into hospital. They gave me a gown to put on and some paper shorts, so I didn't feel too naked! I took my own dressing gown as well, and was able to keep my bra and top on under the gown.
The actual procedure was quite quick. I was dreading it, but when they inserted the tube it wasn't painful at all. I chose to watch the video. I could feel the camera inside me, but it wasn't painful. It was a bit crampy as they went around the 'corners' of the colon, and made me gasp, but otherwise not too bad. It was over really quickly and then I rested in a bed for a while before I was discharged.
I wouldn't worry, it's really not too bad, not nearly as bad as I was expecting.