Matt, the winner of this season's show lost 157lbs and dropped from 40% body fat to less than 3%. It was a rough ride and at some points it seemed like he wasn't going to make it.
BTW, he accomplished the transformation above in six months.
In case you think that Matt's success was a "results not typical" situation, you should know that out of 14 contestants this season, only two did not lose a significant amount of weight. One of them was Nick, who had a bad attitude throughout the time spent on "The Ranch" (the desert mansion where the contestants spent 12 weeks) as well as afterwards when he referred to his time there as "the worst experience of my life". In his post-show interview he spoke about "eating healthy and walking being the correct way to lose weight." The fact that he didn't show up for the finale spoke volumes.
The other contestant that didn't lose much was Kathyrn, who suffered an injury while on the show and I believe would have done much better if she had not been voted off and stayed on longer. She seemed to suffer from unfamiliarity with working out in general and needed more time there for it to become a habit for her.
Everyone else made a phenomenal change in their health and physical appearance, and the majority of them looked so different from their weight loss that they were almost unrecognizable compared to their "before" images.
Although The Biggest Loser does suffer from some cheesy-ness in the production of the show (with "beat you over the head" attempts at portraying drama) overall it's well worth watching. I can feel much more empathy for people who are trying to accomplish something like this than I do for the whiney, backstabbing bunch on Survivor. By the finale I was as proud of this bunch of folks as I could be.
I hope that as they start to do specialized versions of the show (families, couples, etc.) the it does not deteriorate in quality.