i don't know what product you're looking at, WeAre138, but the first slim-fast shake I clicked on had 35 grams of sugar.
http://www.slim-fast.com/products/pr...product_id=366
the second one I looked at had 39 grams of sugar:
http://www.slim-fast.com/products/pr...product_id=346
On the atkins diet, that's a lot of sugar. In addition, that's pure sugar, probably high-fructose corn syrup, and your body doesn't need that. A 40 gram sugar bomb is just going to spike your blood sugar then leave you feeling listless.
I like the atkins brand shakes, but you can also get cheap flavored soy-protein mix in most stores. The best diet shakes for atkins are ones with lots of protein and lots of fiber. It fills you up well without the sugar.
Also, plain water is pretty darned filling too, make sure you get 8 glasses a day and drink a glass 20 minutes before eating a big meal.