I was a competing boxer and a nightclub doorman for many years so I may be able to provide an insight.
I started boxing initially, because I wanted to overcome my fear of being punched in the face. Obviously I have been punched in the face MANY times, more so with a gloved fist, but I have also been punched with bare-knuckles, kicked, elbowed and headbutted.
Does it hurt? Depends where you get punched but usually not. I was quite surprised actually. In the heat of the moment, with your adrenaline pumping you barely even notice.
If it's a hard shot, sometimes you just have a flash of stars before your eyes, of course if you are hit on the jaw, your body has a natural reflex for you to go momentarily unconcious... that hurts when you get up!
The worst knockout is a temple shot or the top of the head... if you go unconcious from one of those shots, your head pounds and you feel sick at the concussion sets it... notice in a boxing match, a fighter dropped from a jaw shot can often get back up and fight and sometimes go on to win, that's because it was the jaw reflex that dropped him and not a concussive, brain rattlin' shot!
If you want instant pain, take a hard shot on the nose. You can't breathe, your eyes water and the pain is sharp and you really want to just run home to momma!
In a fight situation, a body shot is far more painful and inhibits your skill. If you take a hard shot just under the solar plexes, you drop to the floor, your legs go like lead, you can't breathe and you feel like you are going to crap yourself for about three hours afterwards... it's the most draining of shots short of being knocked-out.
A hard punch behind the ribs in the kidneys is also a fight ender. You feel like you've been shot and again, your legs, guts and bottle all go out the window.
A suckerpunch won't hurt until you either wake-up or the swelling comes up.
I've never seen a boxer spit-up blood from being punched in the stomache, or fighters in the street for that matter... it would have to be an incredibly hard shot to either rupture the stomach lining or even worst, force a rib into the lungs... get yourself to hospital if blood comes up for any reason!