I think the answer depends on several factors. Already mentioned is your normal eating pattern at WDW. Two other factors which are equally important, IMHO, are whether you have a car, and the composition of your family.
If you have a car, the best comparison would be to compare how you actually anticipate eating if you don't do DDP vs. the cost of DDP. We come out a lot better eating offsite in my calculations, but your math may be different.
If you don't have a car, you are a captive audience and your choices are limited to DDP and DDE...or just paying regular price in cash. (Keep in mind that you have to be a Florida resident or AP holder to qualify for DDE. If you are not either of those, you would have to add the differential between the tickets you would normally buy and the cost of one AP to the DDE cost in order to get an accurate comparison.)
IMHO, the only accurate comparison is anticipated cash cost vs. cost of DDP vs. cost of cash - DDE discount, less the cost of the DDE. If you make the bogus comparison of DDP cost/menu price of DDP meals, of course you will come out ahead on DDP every time. But that's a Fantasyland comparison.
The 2008 DDP reductions are a big letdown for many families who have enjoyed DDP over the years. Our family has enjoyed DDP, although we are getting pretty tired of the standardized menus. It has not saved us a lot of money, but it has been convenient and we've enjoyed it.
However, with the changes with DDP 2008, we will no longer use it. And, because of not using DDP, we will rarely dine onsite in 2008. Over on the DDP board, I have seen a number of comments from non-DVCers who have previously stayed onsite specifically to get DDP...who now say they will stay offsite specifically because DDP is no longer attractive for their families.
We always have a car, so that makes our choice between eating primarily offsite and using DDE. That's a really easy choice for us -- eat at better restaurants offsite, for less money, and do an occasional character meal onsite paying cash. To be honest, we are really looking forward to eating offsite.
All that said, families with several kids <10 may well find DDP still advantageous.