I had one done and it's wasn't pleasent, but it wasn't painful or anything. Out of all the tests, it's probably the one I'd least like to do over again (with the exception of a colonoscopy). The worst part was the waiting. I had it done in an x-ray/radiology center and what you do is you change out of your clothes and into a robe. Then I had to drink two bottles of barium thru a straw, which basically comes as a powder and they mix water with it and put it in the fridge before you get there. It by no means tastes good, but there's actually a hint of sweetness to it. Drink it fast (of course!!)
There was a tech/nurse guy who was responsible for me, and basically they had me walk around the hallways so it could move through me (he told me "if you have to poop, go poop. it means things are moving"). As far as I knew I was the only one doing this test because I was the only one walking the halls. Every 20 minutes he'd find me and take a quick x-ray, then he'd develop it and see how far the barium had moved. I guess because I told them I had crohn's they assumed they would be done pretty quickly.......but things don't actually move through me that quickly (I'm a once-a-day guy).
Anyway, once they determine the barium has moved far enough, they have a doctor who specializes in taking these pictures come to bring you into another room with a special camera that they aim at certain parts of your abdomen that takes the detailed pictures. After about 2 hours they took me into this room and I thought I was done, but after looking at the pics they told me that they were wrong and things hadnt moved far enough. So then after another hour they had me go back in and they finally got the pictures they needed. They sent the pictures to my GI and I got the results probably about 3 days later.