Behind water and milk, sugar is the third and fourth ingredient of Slimfast.
Slimfast is not much more than sugar water. It will be difficult to lose weight/stay trim drinking Slimfast with any sort of regularity.
The nutritional makeup of quality meal replacement shakes is quite a bit different (and better) than Slimfast.
Myoplex has a lot of high quality protein in each serving, and it is low in sugar and fat.
As is usually the case, with meal replacement shakes, you get what you pay for.