We've visited numerous times -- sometimes on DDP, sometimes using the DDE card, other times just paying cash. I've never noticed any difference in service based on how we were paying for the meal. We've usually had very good service, occasionally outstanding service, and rarely poor service -- but I haven't detected that it had anything to do with the method of payment. It seems to have to do with the individual server having a bad day or a particular restaurant being poorly run.
Most of the servers I've talked to love DDP because it brings in more business and therefore, higher tips. I've heard only two complaints -- neither one of which had anything to do with free DDP, because we don't go at that time of year.
The first complaint, which I've heard several times personally, is that the DDP attracts "a different crowd than we usually get." When asked, the servers have said the patrons tend to be more demanding and less appreciative. I've also seen a couple of examples of that behavior.
The other complaint -- which I have not heard personally, but have read here -- is that some of the non-Disney restaurants don't give the wait staff their full tips. Disney supposedly pays something like $22 per TS to the restaurant, and those restaurants compute the tip on what they receive from Disney, rather than the customer's bill. Frankly, I think that's an issue between the waitstaff and their employers...not a DDP issue.