I've found it especially helpful to see the breakdown of fat, protein and carbohydrates. As a vegetarian, I sometimes worry about getting enough protein, but after tracking my food for a couple of weeks, I've realized that's not an issue. Protein can add up when you're not even looking – in whole grains, cheese and yogurt, and even vegetables. Unfortunately, as I've found, fat can be just as sneaky.
One feature I like is the meal saver, which allows you to create a meal out of foods you regularly eat together (milk and cereal, perhaps), and then quickly add it to your plate in the future. You can also share these meals with your friends if you choose.
My only gripe with the site is that adding a meal made from scratch can be a bit tedious, but the amount of food products already in the system is pretty impressive. There's also a long list of exercises in the system, and adding one to your daily activity log adds to your allowed calories for the day - a little reward, if you will. For me, it's motivation to hit the gym, knowing I can have have a little something extra when I'm done!
Related: Do You Keep a Food Journal?