i personally think that having a slim fast shake every once and a while is better than going to McDonald's when your on the go.
i use the powder, i add in fruit and half cup of milk and half cup of water, add ice and blend.
i mainly use it when my teeth are bothering me so i dont starve.
otherwise you can buy protien powder and add it to fruit to make a pretty healthy shake.
i really prefer my shakes with juice & fruit.
this is my favorite breakfast and snack (add yogurt and oj for breakfast)
its just not filling so you will be hungry afterwards if you dont add in some protien. (peanut butter is a good source if you dont want the powder)
just remember dont do more than 2 meals a day from a shake (unless dr perscribed) because it will be a higher chance of failing due to hunger.
also eat 3 snacks as well as one meal a day if you really want to be successful at this.
i personally am doing pretty good with this plan, and if i dont have shakes one day i keep in my calorie range and i have still been losing weight.
good luck