With the high price of Disney tickets and Universal tickets, I think it's well worth it. It helps us decide what attractions we want to do in the timeframe we have, gives us opening and closing and parade / fireworks times, special event times without having to look elsewhere, notifies us of extra magic hours (means we avoid a park on that day, as we stay offsite, etc.) // I also like their ratings of attractions and ratings by age group of every attraction.
Playing around with my plan helps we get excited about my trip. Usually my 19 year son and I work on it together and both get excited together -- really fun.