I've been using this IPhone app for several months and it is very good for documenting diet, weight changes, nutrition, daily caloric intake. The weekly caloric, nutrient, and net calorie breakdowns are super valuable for understanding just what your eating. It can be crossed linked with fitness apps (I use Runmeter...but I can't cross link it drat!) and device apps (heart monitors for example).
So how has it helped me? Stabilized net calorie consumption, stopped goofy weight shifts from hydration dehydration cycles and pig out cycles (shows up as wild weight swings even if I'm only weighing myself in morning), confirmed that my eating pattern was balanced between fats, carbs, proteins but off on sodium and low on sugars. Most importantly...I know how much food equals energy and days that I don't ride...I'm not constantly stuffing myself. But days that I do ride..I can adjust food in morning and noon to set myself up for late afternoon rides. It could become my most important cycling tool next to the bike and kit.
Eventually it will nail down my basal caloric metabolism (possibly around 1800 calories per day) and help me be a better rider as I age (I'm 58). The added benefit? I'll be way sleeker (maybe I can go sub 170!).