From what ive read it take about a minute for curculation to stop. So you dont instantly black out, however the process you go through would be painful the second it started. So lots of fun pain there. Also, the body and brain respond to pain stimuli even when you are not concious. So yes, still pain until you die. Plus, if you are in open space in a solar system within a certain distance from the local star while it happens you get to experience one half of you boiling from sunlight while the other half freezes.