Regular Slim-Fast shakes are loaded in sugar (40gm). If you were told you can have the atkins shakes, then you can have the slim-fast shakes that are titled Lo-Carb, The high Protein ones will even be okay, but not for every meal.
I like to make my own shakes with fat free - sugar free soy milk (vanilla), whey Protein (25gm protein per scoop), and either a banana or a peach. Put them in a blender and puree it. You an try other fruits and flavors.