ive been fortunate in the two that I've had. Both times the pulp/nerve and shit were basically dead, so I didn't really feel anything.
The first one was because a baseball to the face broke the root of a tooth.
The second was due to an abscessed tooth. That sucked ass. The worst part about that one wasn't the root canal itself which, for me, was painless. What I could have lived without is that the doc had to drill into the roof of my mouth, and also slice the outside of my gum in order to drain the infection and shit that had developed (classic abscess). Needless to say it hurt a bit when it was done, but it REALLY hurt after the anesthetic wore off. I was just moaning in pain, droolin like a damn dog waiting for my prescription, and some dumb fucking bunt took ages to ask about her prescription, consequently delaying my meds. At least he said it was a "beautiful root canal."
Overall, just bring some music, and hope they numb you up real well.
If the pain is mild or pretty much non existent, then the worst shit you should expect is the horrid drilling noises and the feeling of them scraping out your pulp, which is fine and dandy.