I have been using the Lose It! application for the iPhone/iPod Touch for a couple of months now and I say it is a very useful weight loss tool as it allows me to set my weight loss goals and help me carry out any weight loss plan efficiently.
I am currently using Lose It! version 1.2, so I can now modify my program by simply editing my profile (this is very useful for those who encounter dieting setbacks). It also comes with a passcode lock, just perfect for those who don’t really want other people to know how overweight they really are. 
