D.W.
I have not personally used Alli, but I was a representative for Xenical (the prescription version) when it was FDA approved 10 years ago, so I know a lot of the clinical information about the original product. Allis is just a lower dose available over the counter.
Myalli.com is a good place to go for information as it's a social community (similar to Facebook or Mamapedia) for people who are using it.
It works by blocking 30% of the fat in your diet from being absorbed in the intestines......that means, it is excreted in your feces.
The drug will make you be honest about your food intake, or you will have embarrassing side effects. We'd often hear from doctors about patients who ate a fast food meal, took their medication and had an embarrassing event (usually passing gas and leaking fat).
The FDA recently issued some new language around its effect on the liver, so I'd recommend you check it out and perhaps call your physician just to be sure there are no concerns with taking it.
Alli will have NO effect on your appetite (my biggest problem is overeating as well). So, it may not give you the desired results if your problem is portion size. I would strongly advise against anything that boosts your metabolism as most have problems with cardiovascular health.
The only thing I can recommend for portion size is to slow down how quickly you eat (it takes 20 minutes to feel full), drink a glass of water before you eat (to fill your stomach up) and get as much fiber in your diet as possible. Fiber will both fill you up and help block the absorption of carbs (you must have more than 5g/serving for it to work this way).
Good luck!