I did Medifast years ago ... yes, it's like Optifast. I did lose most of the weight I wanted at the time, but got really sick of having just those shakes after about 5 or 6 weeks. And of course, because I wasn't learning new eating habits I put it all back on and then some. I also believe it contributed to the development of gall stones.
If you're deadset on doing something like this, I would say you should do something like Slimfast (though I would recommend more of a protein shake, Slimfast is high in carbs) and do it only for a short time -- say a week or two -- just to get a jumpstart.
But really, I'm inclined to agree with Jenelle -- go back to what you know works. It may not come off as quickly as you want, but in the long run it's got to be better for you.