Medifast (and other meal replacements) were designed for people who have more than 50#s to lose. As they are in the business to make money, these products are now marketed and sold to anyone.
With 30#s to lose, your weight loss will not be as rapid as someone who is much heavier. You could expect to lose about 2#s/week IF you eat only the 5 Medifast meals and the one healthy meal of non-starch veggies & lean protein each day.
It is a very restrictive diet and for it to work you cannot go off eating other things. It is also very expensive, especially if one does not stick to it. Then you have to learn how to eat correctly to be healthy and remain thin.
Dieting is not easy and there are no shortcuts. With 30#s to lose maybe a plan like South Beach might be better? It would cost you less and if you stayed on the unofficial Phase 1.5, your weight loss probably would be as fast as Medifast with that amount of weight to lose. Plus, you would be eating in a healthy way that you could use for the rest of your life.
Just my thoughts. Good luck in whatever diet plan you choose.