I agree with what most of funniegrrl says above, but I do think SlimFast and similar products have their place. Let me explain.
I did use SlimFast for about a month when I started my 100 lb weight loss. And I did it for this reason: I wanted a clean break from my old, terrible eating habits. It was radical departure from the past. Ripping the band-aid off, or diving head first into cold water. I followed the 'SlimFast Plan' and it did work. But I didn't find it very pleasant. After about a month, I constructed new, good eating habits pretty much from scratch. I think it was a lot easier to eat a small sandwich for lunch after doing the shake thing than it would have been in comparison to 'before'. I did not then and do not now view SlimFast as a long term or even particularly quick solution, just a tool I used to make a break from the past.
If you don't need a radical break, I'd just eat portion-controlled, lower calorie food. You can have similar calorie intake with real food, and it is much more satisfying.
Having said all that, if I was doing it over, I would probably look for a product with less sugar and perhaps more protein. It's possible some of the new low-carb SlimFast products fit this bill; I haven't kept up--read the labels. These days I use SlimFast once in a while as an easily-digestible energy drink before a long run. Not exactly the optimal weight loss potion.
And a final note: I think all of these weight loss programs and approaches are just tools. Some may work better for you than others, and what works for someone else may not work for you, but in the end, it is up to each of us to make the decisions about what to eat and how to move. Remember that any of this stuff is just a tool and not a solution; the real solution is within you.
Long enough have you dream'd contemptible dreams,
Now I wash the gum from your eyes,
You must habit yourself to the dazzle of the light and of every moment of your life.
-from Song of Myself, Walt Whitman