I've had a HIDA scan. This was after going to a GI for a swelling abdomen and nausea rather than conventional gall bladder symptoms (pain). My test came back negative and I had a cholecytsectomy (sp?), afterwhich they told me I had a small, benign tumor in my gall bladder. Most people have stones, I think. My surgery was almost a year and a half ago, and my symptoms have still not gone away. Last week I was finally told by another GI that he thinks my stomach episodes are really modified panic attacks.
The HIDA scan itself wasn't that bad. Like you, I was worried going in, but if what my current doctor says is true, most all of the discomfort I experienced was due to an anxiety attack. They'll administer a drug with a needle and that'll make you feel kind of weird for a minute, but again, it's not that bad...just kind of strange. I didn't get sick and it didn't cause me any pain. I would assume that taking this test wouldn't cause any discomfort that you normally wouldn't feel on your own, and the drug is actually to make sure your gall bladder is doing something while they're looking at it. I don't know for sure, though.
If you're like me you're going to worry regardless of what I tell you, but really, it's nothing. The facility I went to had a stereo in the room and even with my anxiety I was able to focus on NPR pretty easily. If you don't like the radio, bring your favorite CD.