I've always understood nicotine hit to be the sensation sought for by smokers: whatever it is that makes nicotine pleasurable to them (I've never experienced that); thus, positive, desirable. Contrary to nicotine poisoning, which produces the symptoms described above—now that I have experienced and it's extremely unpleasant.
If I were to compare it to the naval scale for alcohol effects:
One sheet to the wind = tipsy
Two sheets to the wind = mildly inebriated, happy, speech normal
Three sheets to the wind = very drunk, slurred speech, trouble walking, room spinning around, puking, &c.
Four sheets to the wind = dead drunk (literally even): alcoholic coma
I've rated nic-hit as equivalent to one or two sheets to the wind. Beyond that you're in the province of substance poisoning.
Anyway, that's how I have understood it. Maybe it's time to adjust my definitions.