I know that when I was losing weight, I found that tracking both my activity levels and intake really helped me manage my weight loss. Often if I found my weight loss stalling, I'd check and find that my caloric intake was too low or I wasn't getting enough protein in. When I'd make the appropriate adjustments, I'd get going again. The other thing I found was that when tracking my activity I realized that I needed to up my caloric intake a bit to be sure that I was getting that baseline of 800 to 1000 calories a day to be sure that I didn't drop into starvation mode where your body gets more efficient with less calories.

Now, that I've lost the excess weight and shifted to the maintenance phase of my journey, I find that I don't journal my intake or activity nearly as much as used to. Now, I only do it if I see a trend occurring that I don't like... Either dropping or gaining too much weight. When I look at my logs, I find that I can actually see what I'm doing and when I make the appropriate corrections, so that I'm getting in enough protein, my weight balances out very quickly. 

I have tried Fitday, Sparkpeople and LiveStrong/MyDailyPlate. I really like www.mydailyplate.com the best so far because I don't have to reinvent the wheel when I'm logging my food choices. I like the graphic pie chart that immediately shows me a visual so I can 'see' if my proportion of protiens, carbs and fats are on track.