I followed the mcdougal program strictly for quite some time early in my teens. My whole family did after my mother used it to lose a lot of weight.

It worked REALLY well for me to lose my weight I had at the time. There was no denying it for me.....everybody I know personally who has done it strictly has lost significant amounts of weight.
However, that was the least healthy time of my entire life, seemingly every illness under the sun seemed to flare up with me. My cholesterol and blood pressure was horrid (high high high cholesterol and tryglicerides.....and dangerously low blood pressure at times). My hair, nails and skin were all in bad shape at all times. That time period was when my osteoperosis and arthritis got really bad. My blood sugar was always peaking and crashing, and my moods were getting really bad....I was always extremelly nervous and fidgety (much more than normal even, and I am diagnosed as having OCD, and BP2).
It wasnt sustainable to me, as I never felt well, and was constantly hungry.
Sure the mcdougal plan you dont have to count anything like calories, but it is such a low calorie woe even when you eat until you feel like you can burst, you can still end up ravenous shortly after. I feel strongly that the mcdougal woe caused my already bad issues with food to get much worse....soon after I gave up following mcdougal/ornish, I became anorexic (bottomed out at 130 pounds at 6 foot 3 inches tall before I got help). After that my weight ballooned up to my highest ever of 250 pounds binging on junk. Then my experience with low carb began last year, and here I still am, healthier than I have ever been in my life, and finally seeming to have been able to maintain a decent weight for my height, without obsessing over the food I eat.

*** I am no longer vegan or veggie, I eat a LOT of dairy and eggs, some soy, small amounts of other legumes, and two or 3 times a week I have a small portion of meat or seafood.

Btw my mother who I mentioned did the mcdougal plan strictly for years...but has slowly but surely as of the last year with my influence become more moderate...eating more healthy fats and protien...still vegetarian, and not totally low carb, but it is working for her and she says she feels great now and her blood levels are much better and hasnt gained back any of her weight she had initially lost with mcdougal.