I have not had an endoscopy, but I have had the follow through - aka Upper GI series or Barium Swallow with small bowel follow through.
It really is not that bad of a test. Nothing invasive.
Besides the wait, there was nothing to it. I had to fast for 12 hours previous too, which I didn't mind because I had the test at approx 9 am, so I just fasted from the night before and was asleep most of that time
It is done through x-rays and fluroscopy ("video" x-ray).
At first they gave me some fizzy crystals to drink... this fills your system with air, and although uncomfortable (as in wanting to burp, but not allowed to) it was not painful. Not all places do this.
Then I had to drink a thick barium solution... I cannot think of what to liken it to. It is like a chaulky milkshake... if you can picture that... but it is not completely vile and is managable.
They then take xrays. Some in a standing position, some lying flat and then also get you to turn in numerous ways (roll around) so the barium coats your system completely and to get a better view.
The reason it sometimes takes so long is it may take awhile to travel through your system. During this procedure they "watch" the barium travel through your upper digestive system - throat through small bowel. I suggest you bring a good book, or a mp3 player to keep you entertained. I was there for almost 3 hours I believe... but some people have been there longer. It all depends on your system.
Then there is the matter of ridding your system of the barium afterwards. Be sure to drink plenty of water to try and move it along, as the barium can "cement" in your system.
All in all, for both of these tests... most people here will tell you that the anticipation and the prep (not required for all tests) are the worst part. I was somewhat nervous before hand, trying to imagine what it would be like to endure... but truly it is not that bad. Uncomfortable as in "don't/wouldn't want to do this everyday"... but not painful tests.
Be sure to ask doctor lots of questions if there is anything you are unsure of.