You just never know how much - or how little - it's going to hurt! I've had 9 fillings removed and replaced with wither crown (3) or inlays (6). In the process, I had to have 2 root canals (which I did on the same day). Now, I KNOW my body needs more anaesthetic than normal people, and I ask for it up front. As a result, I don't end up with much pain. Never used the gas, though.
Almost done, GD. I can tell you that, being done is a great feeling. I am finially there!