I have never taken SN, so I can't speak from experience. But from what I have read - and I have read quite a lot on this site - it's not painful, or at least not significantly painful. The vomiting may be unpleasant, but most of us have vomited on occasion, and it's not that bad. From farewell threads, it seems that people start to feel "not quite right" after a short period. Some of them say they are going to lie down for a while. Sometimes that's the last they post, and I presume that they died soon after.
I think that people go unconscious fairly quickly, and if they display what seems to an onlooker to be rather distressing symptoms - which apparently does happen sometimes - by that time they are not aware of those symptoms themselves.