You just have to watch the calorie intake. They are designed to promote muscle recovery and lean growth. also a higher protein to carbs ratio promotes burning of existing fat on your body. There are actually "diet proteins" on the market but they are all a muchness as long as you are adjusting diet to add the calories into it. If you are exercising, having 3 healthy meals, and the lower calorie protein shakes, you shouldn't have any problem dropping weight.
I usually take one in the morning, then if I have an exercise session I take one straight after. (mornings and straight after exercise is when they are most affective) You can take up to 3 a day but I don't overdo them because I'm trying to lose weight, and too many is wasted calories for me when I can be eating real food :P And they are flippin' expensive too!