Okay my honest opinion - it's a load of crap!
Of course depriving yourself of calories works, and the simplicity of it (2 shakes + reasonable dinner) appeals to a lot of people.
Now, if you really want to follow something that is 'simple' in terms of just having 2 shakes, I highly suggest making your own alternative drink. Slimfast is just packed with junk, the same amount of processed sugar as a chocolate bar.
Here's one of my favourite drinks I make:
1 cup mangos, 1 cup skim milk, 1 scoop protein powder, 1 tbsp flax oil, 1 tbsp ground flax, cinnamon
(Your carbs- mango, skim milk, with 25-30g protein in the powder, plus 10-14g healthy fats in the flax oil) ~ it's a lot healthier and tastes even better.
There are SO many combinations you can do. Good luck