I've been to WDW three times during
free dining...twice last Sept and just last month. The times in Sept were terrific. The crowds at the parks weren't bad at all. But...last month was a nightmare. Why? Because schools still hadn't gone back in a lot of states, so families were making one last trip to WDW to take advantage of free dining.
Usually, with free dining, you get the guests who normally spend less on dining, booking more expensive places to eat. I know that when I was at Coral Reef, with no free dining offered, it was quite pleasant. But, when we were there last Sept, it was miserable. Tons of families with young children..it was crowded, noisy and just not all that great. I have nothing against families with young children..don't get me wrong. But usually, if money is any kind of issue, you will find most families eating at buffets or at counter service in order to save a little bit. With free dining, all bets are off.
I really don't think that if school is in session, you are going to get the huge crowds that are normal in the summer when free dining is offered.
I was there for that first week of free dining (I didn't have free dining) and I'll never do it again. At least not during a vacation period. The parks were a nightmare. I have never seen so many people in MK during mid-August as I did between Aug 16 and 21st!! And I've been there at that exact same time 4 times now!! But free dining was never offered that early before..usually started the next week.