I had this done a couple of months ago.
I started with a normal colonoscopy, but the doctor couldn't get around the bend in my splenic flexure (I have a very tight corner there, and the scope was bowing and causing me lots of pain, even though the doctor was experienced and I was sedated and given pain meds). So they stopped the colonoscopy and sent me to the imaging center for the virtual colonography.
Since I was still halfway sedated from the regular colonoscopy, I barely remember it. My first memory is going into the CT scanning room and wondering how I got there. And then I remember when they pumped me with gas to inflate the colon, that it was quite painful - like major gas pains. But it only lasted a few seconds.