Interesting thread. I used IP to lose the weight. I used the maintenance to keep it off fully for 8 months and mostly for over a year. IP products may not be the best thing for a person but most of us do not become significantly overweight because we are choosing to eat the best thing for us

IP is great for taking dieting in baby steps. You don't have cook EVERYTHING just veggies. But if you take the time and experiment with recipes and cooking that one meal per day it is much easier to expand that habit into other meals once you've hit maintenance.
It reorganizes your eating schedule (I'm sure medifast does this as well) into eating 3 small meals and 1 regular meal. If you stick with something akin to this schedule in maintenance it does keep the weight off.
Really the diet is a reset. Smaller amounts. Orderly eating ( measuring, timing etc). It is much easier to stick to a maintenance plan when you already have some of the habits in place such as cooking, recipe searches, orderly eating. Than it is when you try to attack it while dieting. There is the added motivation of maintaining your newly svelte self and one is much more in control of their diet when they are firstly in control of portions. Second to this is quality of the portions. Yes, in the long run (see: maintenance) making healthy (unprocessed) food choices is imperative. But it is one step at a time to get to that point.
Both medifast and IP are processes and neither is something that is healthy to live on for life. They are both but a means to an end. Good luck with your choice.