I'm going for 7 nights in October with DD15 and DD13. I earn Disney gift cards through rewards programs so they are "free" and use them to pay for our dining. We have the following meals planned..
HDDR (Cat 1), Jiko dinner, Yachtsman Dinner, Sanaa lunch, 50's PTC lunch, Splitsville (DVC Member night dinner), and Boma breakfast. The total cost for those meals (including gratuity and my TIW discount) is about $875. I have another $350 budgeted ($50/day) for counter-service meals and snacks since we eat very light around our "main" meals and we will have breakfast each day in our room. That brings our total dining cost to about $1225. I plan to bring about $1250 in gift cards so our dining will be "pre-paid".
Now, compare that to the dining plan. We wouldn't be able to do the Splitsville member night, but we could still eat dinner at Splitsville (1 credit). In order to eat the same meals, we would have to do the DxDDP since we have 10 TS credits worth of meals. For 7 nights, the DxDDP would cost me $2441.25...and that does not include the tips! So I would have to pay TWICE as much money to get the dining plan and I would not get anything more than what I already need. Yes, I would get a LOT more food...but I don't NEED any more food. I see no reason to spend an extra $1500 just to run around trying to get my money's worth on extra ADR's or eating myself silly on food that I might not want.
I would suggest looking at the TS meals that you would like to eat. Look at the menus and choose what you would realistically order. Put together the budget and then see if a dining plan would include those meals. Then compare the prices. If you can come out ahead by purchasing the dining plan (not likely unless you have children under 9 and your doing buffets), then do it. More than likely, you will find that you will come out ahead by buying Disney gift cards and using those to buy your meals a la carte. (Check out the Disney Gift Cards thread here for ideas on how to get cheap/free gift cards) If someone gets sick or just isn't hungry, you don't waste credits. When we have money left over, we let the kids splurge on an extra souvenir or treat before we go home. If there's a lot left (on my last trip, I had over $100 left!), I just keep it until the next trip.