You must, must, must book the dining plan or you will be very sorry. I have been to Disney 8 times and it will be my 9th time coming up at the end of August. I have never been on the dining plan but since I know what restaurants to eat at, I will save MEGA money! If you go to "All Ears Net" (it won't let me post the URL) and look at the menus it will help you out a bit. Granted, I haven't eaten at the restaurants by any stretch but a prime example would be Le Cellier (Canada at Epcot), the entrees alone cost somewhere around $20-30 dollars, plus you get an appitizer and desert and non-alcholic drink and includes tip! I would say that meal alone would cost about $50.00 US per person. I would highly recommend calling for reservations for that restaurant. For my upcoming trip I have already made dinner reservations for 4 out of the 6 nights we are there! A few restaurants I would recommend:
Boma (Animal Kingdom) - buffet with African food (try the carrot ginger soup)
Ohana - Polynesian style (skewers of chicken, beef, shrimp, pork... YUMMY)
Le Cellier - (Canada @ Epcot) Steakhouse
Alfredos - (Italy @ Epcot) Italian Food
Cinderellas Castle - Magic Kingdom (see menu at all ears net)
The Rose and Crown at Epcot is pretty good and is more pub fare like bangers & mash, fish & chips etc. The restaurant at the Mexican pavillion is also very nice and I would recommend sitting by the water near the ride rather than the market area. I haven't been to Teppanyaki or Nine Dragons (haven't heard good things about Nine Dragons).
Hope this helps!