Your post doesn't mention it, but I am assuming you are working out for an hour plus to achieve those kind of figures.
Longer workouts can be muscle wasting, as your body starts to look for alternate sources of fuel, and can break down muscle tissue to liberate protein. As muscle burns calories even at rest, this can hurt your longer term performance and weight loss.
Unless you are training for a specific endurance event, you are probably better off keeping your workouts to 30-40 minutes, and adding more intensity to your workouts rather than more time. Working out harder and getting your heart rate higher creates more health and fitness benefits.
Also, if you are working out 5-6 times per week, burning 500-700 is probably at a level where you need to start eating more to support that level of activity. You will likely max out your weight loss in the 300-500 range without having to eat more.
I'm not saying that 500-700 is bad for you. Just that a slightly different workout program might be more time efficient.
And when it comes to my own exercise, I pretty much ignore my own advice. While I am generally in the 300-500 range during the week, on weekends I like getting on my bike and riding 2-3 hours and burn 1000-1500 calories. I justify it on the basis that I enjoy it.