I had it done a few months ago after they couldn't complete the normal colonoscopy on me.
I was still a bit loopy from the colonoscopy sedation, but from what I remember it was a pretty quick procedure. You definitely feel some gas pain when they pump you up, but it only lasts a few seconds while you hold still and they get the images. Then they "deflate" you, you turn over, they pump you up again, and they inflate you one more time for another scan. Overall it wasn't too bad, just some quick discomfort from the gas.