I've had two.
The first was on a tooth that had already died. My previous dentist screwed up a filling, which resulted in the tooth getting reinfected and dying. I went to this dentist on Thursday - by Sunday night the tooth was dead. Was at the endodontist on Wednesday, and he touched a swab dipped in liquid nitrogen onto the tooth. Didn't feel a thing. So he proceeded to drill, and you could actually smell that the tooth had started rotting inside.
They usually prefer you to go for 2 visits, because they can make sure that everything is fine. They put some antibacterial stuff into the tooth, and they need to make sure it is sterile in there, that there is nothing left living inside. So that is why they like two visits. Other thing to note is that if the tooth has an abscess, it could be really painful. Apparently abscesses contain acids that neutralize anaesthetic, meaning novocaine does not work. So, if you need a root canal, do not wait for it to get worse. Get it done now.
My second root canal was done with my new dentist, who is much better than the old one. He was going to replace the filling, but when he drilled in, he could see that the tooth was done for. It was bleeding inside or something - not a good sign. So, he gave me a few more injections and then started the root canal procedure. Because of all the novocaine, it was fine. I've just had the tooth crowned.
Oh, something else to be aware of. If you have a root canal done, in most cases you will need a crown on the same tooth. The reason is that a root canalled tooth is now dead inside and quite brittle. Without a crown, sooner or later, it will crack.