Those of you who have never used FP+ before may be surprised at how much more spontaneous you can be with Fastpasses on the day of. You don't need to be savvy with the app, either. At any FP+ kiosk in the parks, cast members will let you know what's available and you can secure a fastpass right away. Imagine being able to get a fastpass for Radiator Springs Racers at 7:00 at night. Today, that's just about impossible. But with FP+ you can change your fastpass selections, so windows are opening all the time. That lets the spontaneous people benefit from other guests' cancellations.
I was a big Next Gen skeptic, and I still wonder whether even the great result is worth the exorbitant cost it took to implement. But I was absolutely shocked to find how much I love the new system. I have zero, and I mean zero regrets about FP+ replacing the old system at WDW.
EDIT: One day, I rode Peter Pan's Flight on a whim. This is a ride whose wait times are always too long for me to justify waiting in line. But on a whim, I decided to check on fastpasses for it. Twenty minutes later, I walked right up the fastpass queue.