USNews covers recent research that calls the effectiveness of dieting into question.
Slentz, Hawks, and other researchers note that most studies show that the vast majority of people can't stick with a diet very long. Though some dieters do make lifestyle changes that lead to permanent weight loss and better health, most regain much, if not all, of their lost weight in three to five years. Results reported last week from the federally funded Women's Health Initiative do little to enhance dieting's reputation. After seven years, women on a low-fat diet maintained a mere pound of their initial loss. And some studies show that frequent dieters actually gain weight.
The article goes on to suggest that healthy eating and exercise habits are more important than actual weight, which I think is probably true. I also think that it's far easier to establish a new set of habits than it is to stick to some prescribed eating plan.
Update: As a follow-up: Short-Term Lifestyle Changes Improve Health Even Without Major Weight Loss.