To be quite frank, FitDay is really late to the iPhone party. I've been a FitDay user for 5+ years and hoped there would be an iPhone app long before now.
I've been using the iPhone app called LoseIt. It does not have near the functionality of FitDay, but it works well, they are updating it, and I can record my food and workouts with it.
Unfortunately, I've almost completely stopped using FitDay because I'm not going to record the same information in two different place.
I'll bet I'm not alone.