Nicotine is highly addictive, so there is obviously some impairment from it's use. It isn't the same as alcohol or other drugs, but it still affects brain chemistry, health (to a degree), and the behavior of the person addicted. Those of us who are used to consuming it regularly don't necessarily notice the "high", but we certainly notice the withdrawal symptoms.
In addiction circles, 'sobriety' tends to mean complete abstention from the substance the person is addicted to. For example, an alcoholic can't have one or two drinks even if their function is not impaired and still be considered 'sober'.
Obviously, the OP isn't referring to nicotine addiction, which is fine and good. It is almost certainly the lesser of the evils. Surf is correct in stating that unless someone is free of all addictive substances, they aren't completely sober. They may be sober from alcohol or drugs, and that itself is an admirable accomplishment. Yes, it may be splitting hairs, but Surf is technically correct.
With that said, congratulations and best wishes to all who have achieved sobriety- no matter the substance involved.