I just spent a great deal of time analyzing the pros and cons of
DDP for my sister's upcoming trip. You need to know that the rules have recently changed and now a gratuity of 18% will be charged to group of six or more. If you have fewer than six, I assume you pay a gratuity based on the value of your meal as you would normally. DDP has never covered alcoholic drinks and some specialty drinks.
There are several plans with slight differences. How many days you can get depends on the length of your stay. A four day stay gets you a plan for 4 days. There are several plan levels, but basically you choose to have one sitdown per day, or just 2 quick counter service meals per day. The other plans just change the number of snacks included, a refillable mug, or even pizza service included in the higher level plans. If you don't plan to do too many sit downs then I highly recommend just the quick service DDP. You can use these plans at all the resorts and in the parks where counter service is provided.
http://www.wdwinfo.com/disney-dining-plan.htm
Using the DDP really depends on how you plan to eat on your vacation. If you are staying at a site with a full kitchen, then you can save a lot by eating some meals in your room and saving your money for sitdowns.
My sister's family really enjoys the buffets (Cape May, Park Fare, ect..) and since they cost $40 per adult (ages 10 and up in Disney's world) they figure for an extra $10 for the plan they get a quick service meal and snack.
Meal credits do not have to be used everyday and you have until midnight of the day you check out to use your meal credits.
IMHO, since gratuity is no longer included, I think its a close call as to whether you will spend more on the plan or on your own. We came up with almost the same amount, but my sister decided it was peace of mind and less aggravation to have to hassle or worry over prices and just tell the kids to order what ever they wanted.
If you have young children then the DDP is very reasonable, only $12 a day. An adult is now $45.99 per day of your stay. Everyone in your party must purchase the plan for the full length of your stay.
I don't think you can really go wrong either way, its really more of a matter or preference in how you eat and how much you want to save money on your own. Good luck..