I really don't think nicotine by itself is "addictive," I think it somewhat "dependence producing," the same way that caffeine is, or it would not be sold OTC; I won't get into sugar because that's a whole 'nuther can of worms. When nicotine is consumed in combination with the other tobacco alkaloids (including MAOIs), it becomes rather more truly addictive, but I think the level of dependence/addiction will vary according to individual metabolism, susceptibility to the "addiction disease" in any form, and the delivery method. Cigarettes are really the perfect storm of all the necessary ingredients for strong dependence, even true addiction, since many people continue to smoke until it leads to their rather early deaths, even though they know the risks, they just can't quit. Some 20% if I remember correctly.
Just to judge from the accounts around here of those who find themselves decreasing their nicotine levels because they become more sensitive to the more uncomfortable effects of nicotine -- their tolerance is actually decreasing. That doesn't happen with caffeine.

And I don't mean those who go from cartos to sub-ohm RDAs, but those whose hardware remains of more or less similar efficiency, but just can't handle that much nicotine anymore. I think that's one of the best secrets about nicotine; that tobacco we smoked had been "doctored" to make our bodies overlook how toxic it is, but when you remove all that "doctoring" and consume just the nicotine, your body is no longer fooled; your natural response to toxins begins to manifest, as the effects of the tobacco gradually wear off. It's brilliant.
Andria