I really like Botox for upper lip lines. It works by keeping the muscle around the mouth from "puckering" - which is what causes the lines in the first place - and why they call them "smokers" lines (since that is what you are doing when you are smoking - which, as we all know, is a bad idea from both a health and a cosmetic perspective). In fact, you should avoid activies that cause your lips to pucker - like drinking through straws, blowing on hot foods, whistling etc.
As far as side effects, I tell my patients that botox in the upper lip may make your lip feel "clumsy". Meaning that you may notice it is a little difficult to drink (especially through straws - which you aren't supposed to do anyway). Most patients get used to the Botox and are not bothered by it. About 20% of my patients dislike it enough that they don't want to do it again. Of course you can get temporary bruising from the needle pokes. The dosage should be low (2-6 units for most people).