I've had a few done. Not a big deal at all. In fact, even if you didn't need a root canal, you're hypothetically better off in the long run having it done since you don't really need the nerves in your adult teeth. You basically guarantee you'll always keep your real teeth and keep them pain-free for the rest of your life. Not a bad deal.
The worst part is the novocaine injection, and nowadays they even rub some kind of mint-flavored numbing gel on your gums right before the give you the shot, so the pinch you feel is minimized. Otherwise, it's no different than getting stitches somewhere on your body when there's a novocaine injection. You shouldn't feel any pain. Hell, you could saw your entire arm off and not feel anything if you were shot up with enough novocaine beforehand.