calorieking.com is a good website to use with FitDay. It has a lot of fast food and junk food, and it gives you a nutrition label similar to the one you fill out at fitday.com when you're creating a custom food.
Although FitDay probably isn't that accurate, I have learned a lot about my eating habits. There are a lot of foods that I didn't realize were as unhealthy as they are. I've also started paying attention to portion sizes, which was my biggest problem before. But one of my favorite features is the pie chart that keeps track of the percentage of carbohydrates, proteins, and fat in your diet. I was eating WAY too many carbohydrates before, and I think I was getting a lot of headaches because of that.
The only problem I have had with watching what I eat so closely is that it has become extremely obsessive and I spend way too much time thinking about food and weight loss.