Through a separate weight loss program, I did Optifast 800 for 4 months total, 5 shakes or meal bars a day with nothing else!
Had great success with it, lost 90 pounds in that 4 months. However, once it was over, the weight came back quickly. So here I am LOL.
Stick with it though, I found that after about the second week your body really adjusts to the lower calorie intake. Just drink more water and you should be alright!