The earliest ressie you can get is 8:05. They will let you in the park right at 8:00, so there's not much time to leisurely stroll down Main Street if you want the earliest time. Getting one of the first spots is wonderful because the food is very fresh, and there aren't as many diners, so the character interaction is wonderful.
When we do this, we are done with breakfast about 8:50, and that's when the real benefit starts:
We have left this restaurant and gone either to the rope drop at Adventureland or Fantasyland. When we were at the Adventureland drop, we were the first ones escorted back to Splash Mt., and got special recognition as the first family (a certificate and a free picture from the ride). We also got escorted up the exit ramp at Big Thunder Mountain, and they put us on very quickly, because waiting for the picture took a few minutes. This was a great experience!
When we went to Fantasyland, we were able to do the entire section and get fastpasses for Space Mountain all in the first one hour in the park. To me, this was totally worth getting up early and paying for the breakfast at Crystal Palace.
Going back in February, and will do this again.