I had one on an incisor as a teenager (a wrestling match knocked it loose and it formed an Abscess). It isn't exactly fun, but it won't hurt much except the needle at the beginning. I was only under local, but i hear sometimes you get the option to get laughing gas, I would take the gas if you can. It was significantly more painful before the operation than during. The vibrations are pretty intense at times and you have to stay in a pretty uncomfortable open mouth position for an hour (although they prop your mouth open for you). My least favourite part was when they clamped a uv lamp around the tooth, which felt like a big pressure for me. My mouth and tongue also got very dry, and the squirts of water they provided didn't do much to make that better. I don't remember much of the after care but it wasn't bad.

10years later fwiw, my root canal developed an infection and I needed dental surgery to clear it out. During this the local numbing was ineffective for some reason and the dentist had to go in 3 times to try and renumb it, but it never really worked. That really really hurt, but it is not exactly a typical case.