I lost 12 lbs in 5 weeks using caloriecount.about.com. I used 75% of kilojoules from my PowerTap to come up with calorie burn per ride. I created my account as sedentary to hone in on base burn rate, then just made sure to keep it below 250W on my rides so I was mostly just burning fat and wouldn't replace the calories. Caloriecount's prediction chart was spot-on with my rate of loss.

I didn't weigh food, but just looked things up and used nutrition info on products. I also simplified my diet so I was eating a lot of the same things over and over again, so my tracking didn't have so much variability.