First you have to get an idea about how sensitive to it you are, so start slow. I would start by taking 1/2 of a pill 2-3 times a day for the first couple of days to see if you are sensitive to it or not. If you are then 6-8 mg per day max. Sounds low, but trust me you do not want to over do this stuff, ever. If you are not sensitive or build a tolerance fast, then you can take up to 4 mg 3-4 times a day.
As far as results go: it burns some fat, helps your workouts because it increases the airflow to you're lungs, you might get a little stronger on it and supposedly it helps with cholesterol issues, so that's about it. It is mainly used for fat burning.
Possible side effects: increased heart beat (allmost a sure thing). Higher blood pressure, panic attacks. The most serious possible side effect is ARYTMIA. If you take too much too fast it can increase your heart rate way too much and can results in arytmia, if this happens you have to go to the ER ASAP!
Bottom line: It's a good product, but start taking it slowly and don't expect any ultra dramatic results just from taking it, diet, cardio, weights come first and after that drugs.
Good luck!