I lost 20 pounds with SlimFast about 11 years ago, but that's when they had the microwave meals, soups, snacks, and the whole nine yards.
What is it that you're tired of with Atkins?
I would bet that if you switched from a low-carb plan to SlimFast, you would initially gain weight... all that sugar and those carbs in the shakes would certainly make you retain water at first, if not feed your fat cells somewhat.
If you really want to go to a shake diet (which is a quick fix, not a permanent fix...), why not stay low-carb? Carb Solutions and EAS both have good pre-made shakes, and there are a ton of flavors/brands in powdered shakes... they can be spiffed up with a little heavy cream, Splenda, DaVinci sugar-free syrups, or even berries. And, of course, there are more bars than you can count (though a lot of those leave something to be desired...).