I use the LIVESTRONG app. It does a decent job at diet/food tracking. It syncs with the info you put into the web site as well. The app seems to have a fitness tracking as well, but I have never used that... I am too lazy to transfer any workout to the ipad later.
I've been using the 'Lose It!' App for over a month now and have lost 10 pounds (I started at 190 and am down to 180--losing 2 pounds per week as I had programmed into the settings)...this is app will really work if you're very list-minded and don't mind doing a little extra homework (googling calorie contents occasionally)