The simple answer is that the meaning is the same (that at some time in the past, you were a smoker, but you no longer are), but because the verb is "use", and its past tense is "used", it follows that I used to smoke must be grammatically correct. The uncertainty probably arises because "used to" and "use to" are homophonous.
Interestingly, this "use" has no present tense, so although the form "use" appears to be the present form, it is in fact the plain (infinitive) form, which is correctly used in negatives and with inversion: I didn’t use to smoke; Did he use to smoke?
There is the added complication that "use" can be a lexical verb or an auxiliary, though the books tell us that most speakers treat it as a lexical one. I think that’s because of the unacceptability for most of the auxiliary use found in: Used you to smoke? and Smoking usedn’t to be allowed.
(You edited your question to ask about conveying current habits. This "use" has no present tense, only plain and past forms, so to indicate that you smoke now, you'd have to say something like "I'm a smoker", or just "I smoke".