If the room is more than $40 off, then take a room discount. For a family of 4 (like ours) the plan would cost us $160 a day. Really we won't even bother anymore - stayed our last time in one room a year ago, and it's just too tight as the kids get older, so we're not even considering an on site stay anymore. But, if we got a room at the value resort for about $100 a night, and the dining plan was free, that would be a bargain. If we got a room discount at a deluxe resort and it was saving more than $160 or so, then that would trump the free dining.
If you can't get a good discount on the room but free dining is offered, I guess just figure that it's worth the cost of the DDP.
Going solo, I would rather be free to pick and choose, and forgo the plan.