Whenever I drink Slimfast (as occasional meal replacement, not part of Slimfast program), I find that if I drink a large class of water with the shake, it helps me feel fuller for longer. If you feel hungry, sometimes it's your body's way of saying that it's thirsty, not hungry.
The main reason why Slimfast is successful is because you're reducing your calorie intake. There's nothing medical/magical about the ingredients in the shake. Weight loss is a simple rule of math - consume less than you burn. If the shakes work for you, that's great! but, if you find that you're hungry or have any other funny side effects from limiting your diet to a dairy-based shake, try finding food options out there with similar calorie values as the Slimfast shakes/snacks (some Lean Cuisine meals or even a homemade sandwich with low-cal bread, low-cal cheese, healthy choice meats, etc.)
Good luck! Weightloss is a hard journey, but well worth it in the end.