FitDay helped me drop 30 lbs in six months last year. I used it less religiously Jan - May but even putting in my numbers a couple days a week helped to keep me focused. I only gained about four or five pounds back before dropping it again when the weather allowed almost-daily cycling again.
As said above, take FitDay's calorie numbers with a grain of salt. I enter my weight about 5 pounds below actual to bring the daily normal use more in line with other calculations I've done. If I have any question about food, I always over estimate (is this apple a medium or large? Large.). For the activities part, I use a HRM to get the calories burned and then just fiddle with the time in FitDay so the numbers match. Every activity I've used in FitDay over-estimates calories burned (cycling, mountain biking, running, tennis, hiking, swimming, lawn mowing, racquetball, basketball, etc.).
But, with those "customizations" I dropped a consistent 5 lbs/month for six months. So I got the numbers to work out well.
And I very much agree about the nutrition portion of FitDay. While I could probably use Excel to just count calories FitDay has made it much easier to look at fat/carb/protein ratio, track vitamins and minerals, and limit sodium and cholesterol. I have no proof of this other than my own personal experience, but I think that the nutrition part of FitDay helped me to keep up a large physical workload and improve biking performance, while still dropping weight. I think that if I just limited calories without paying attention to the rest of it I would have a harder time building muscle and recovering.
I just use the on-line version so that I can update things whether I'm at home, at work, or on the road without my own computer.
Most embarrassing custom entry in FitDay: Doritos.