Which one are you using? I've been looking at getting the Fitbit Force, which can track elevation as well as steps counted. Right now I'm just doing the one-two method of logging my aerobic exercise in Runkeeper, then tracking food on MyFitnessPal. The main problem with this is, Runkeeper doesn't give me an overall of how I'm doing, only what I log, and MyFitnessPal is heavily biased towards stuff that has nutritional information on the label i.e. processed food. Sure this pasta dish I ate at a fine Italian restaurant is great, but they don't have to publish nutritional info.
I'm hoping that Fitbit could do a good job of unifying these two systems, because keeping track of both is a pain. Also if Fitbit has any kind of decent weight training logs that would also be good. I use a different program for weight training, but it has no API hooks and doesn't communicate with other apps, so it's like it's own system. Ideally, I'd just love a single app where I can sit down and monitor my progress across cardio, lift and diet.