I had one a few months ago. Ask for sedation and the procedure itself will be very easy. They were halfway through before I realised that they had started :-)
They gave me the results straight away and mine were all clear.
The prep wasn't enjoyable but wasn't awful. I had to drink 2 litres the evening before and 2 litres in the morning. I stayed VERY close to the toilet but walking around helps things go quicker. I mixed my drink with lime cordial to make it taste less horrible. Be careful not to chill it because that volume of cold drink will cool your body too much.
A tip I recommend is to buy baby wipes and nappy rash cream. You will want something kind and soothing on your bum once the prep starts working.
I worried about the whole thing way more than I needed to.