The problem with SlimFast can be summed up in one word: "sugar". That's where most of the calories in a Slim-Fast product (bars OR shakes) come from.
I *have* looked at the Slim-Fast website and seen their basic diet plan...let's take a look.
Breakfast - Slim-Fast shake or meal bar
Snack - fresh fruit, nonfat yogurt or 1 oz low-fat cheese
Lunch - Slim-Fast shake or meal bar plus a piece of fruit
Snack - Slim-Fast snack bar, veggies or low-fat soup
Dinner - 4-6 oz lean meat, fish or poultry; baked potato; veggies, salad and fruit
Snack - Air-popped popcorn, fat-free pudding or pretzels.
The snack bars (according to the Slim-Fast website) average 120 calories, with anywhere from 12-22 sugar grams...which means that of the 120 calories in the bar...48 to 88 of those calories are pure sugar. Additionally there is only ONE GRAM (or less!) of protein. I can't stress how BAD sugar is if you're trying to lose weight - because for many people, eating sugar creates cravings for more sugar - unlike protein, which has a MUCH higher satisfaction rate than sugar.
The ready-to-drink shakes appear to have an average of 220 calories and 35 grams of sugar - in other words, 140 of those 220 calories are pure sugar. (the powdered mix has less sugar, but remember - milk (even skim milk!) has sugar in it as well - 15 grams per cup!)
The meal plan above (taken from the Slim-Fast website) is very low in protein and fat, very high in simple carbs (i.e. sugar).
Now, I'm not knocking the use of meal replacement shakes - just the Slim-Fast type. Understand that the Slim-Fast company are basically selling a product that is VERY cheap for them to produce. Sugar is a very cheap food source. (and I use the term 'food' very loosely in this instance).
Now, if you wanna go the shake route to help in your diet, there are PLENTY of meal replacement shakes that are MUCH higher quality than this sugar water (I call it "SlimFat" myself). They're a bit more expensive - but if you shop around on the Internet you can find great prices (don't buy them at GNC - that's the most expensive place around!!!). My favorite online shopping places for meal replacements are vitaglo.com and netrition.com. And you might find that the higher-quality meal replacements taste BETTER than SlimFat, er SlimFast

Also, the ingredients in the products listed below are of MUCH higher quality.
Recommendations:
EAS' Myoplex Lite shake powder - I like the Capuchino Ice flavor the best...there are a ton of flavors though. 190 calories, 25 grams of protein and less than 2 grams sugar!
Labrada Lean Body For Her - I believe this only comes in two flavors at this time - Soft vanilla ice cream and chocolate ice cream. 170 calories, 30 grams of protein and 1 gram sugar.
Met-Rx Lite Drink Mix - I have not actually tried this product so I can't tell you how it tastes...but it has 180 calories, 25 grams protein and 3g sugar.
Biotest Classic GROW! Meal Replacement - Personally, I didn't care for the flavor but a lot of people love the stuff - they say it tastes like cake batter...judge for yourself...rather than single-serving envelopes it comes in a tub, so you can choose your own portion size. 3 scoops is 300 calories, 40g protein and 3g sugar - I'd probably use 2 scoops as a serving.