We were in WDW in January last year before the dining plan came on the scene, and even though it was a very quiet week attendance-wise, there were still some restaurants we were very happy we had priority seatings (now ADR's) for. The ones that come to mind are Cinderellas Royal Table (dinner), Chef Mickey's (breakfast), Crystal Palace (breakfast) and 'Ohana. At these restaurants walk-ins were either being turned away, or quoted very long wait times.
I can only imagine that with the new dining plan it will be more important than ever to make those ADR's. We have them all made for our upcoming trip with the idea that we can always cancel if we have a change in plans. My husband wasn't crazy about the idea of planning in advance where we were going to eat, but I know he'd be even more cranky if he had to eat chicken strips all week!