Longitudinal studies that have been replicated around the world do present powerful evidence that controlling one's impulses has wonderful benefits.
However, we should also note that there are many psychological barriers that need to be overcome in order to improve one's willpower. This is not a trivial task.
Oh, The Temptation at Vimeo for another video of kids being tempted by marshmallows.
- An updated marshmallow experiment suggests that many of these behaviors are due to environmental factors.