I've had two so far, with more to follow. I'm not sure it's painful, more like intrusive and extremely uncomfortable.
I recall the first time I was sitting in my surgical gown in the ward and listened to a man gagging loudly for about fifteen minutes. It felt rather like I was waiting in a slaughter house.
It is a very unpleasant experience, with a tube down your throat, lying on your side. There is an overwhelming sense to gag but you control this with your breathing. They say it's only for about 5 minutes but it's more like 15 and feels like hours.
You have the option to either have a throat spray anesthetic or sedation. Next time I'm going for sedation.