Disney failed this test for us. I just got back yesterday from our most unmagical vacation ever. Yes, crowds were heavier than expected, and we got caught up in that crazy DHS day where FPs were gone by 9:30, but there were plenty of other problems related to MB and FP+.
Charging to MB failed twice for us, once when we already had trays of food. We tried to call Guest Services from the quick dining location but there was no answer. I had to run to my room for cash and then to guest services at our resort only for them to tell me "Sorry, it sometimes happens" and there wasn't a problem at all. The other time we were in the parks and just left our purchase behind. After that, we always carried other forms of payment. Our dining servers had negative comments about it not working well for them too.
We had constant "blips in the system" where
MDE wasn't working, and rescheduling FP+ was hit or miss. We missed almost one FP+ a day because of ride closures or schedule problems. We laughed every time the system wasn't working. At Animal Kingdom, we had to go to a kiosk because MDE wasn't working and they said they also had problems all day. We did switch to an alternate time, but it didn't fit with our plans, so we skipped it.
We might have been unlucky and found a lot of ride closures, but that was a bummer too. One night at MK we had FP+ for Big Thunder but it was closed so we crossed the park to SM for our other FP+ and it too was closed! People Mover? No, it was down!! So we waited for our last FP+ for MSEP and at 10pm, they announced the parade was delayed. We went home. Now, I'm not blaming the ride closures, but when we had planned them for the end of the day and had no chance to return, it made the night very sour indeed.
We decided that FP+ might work very well if you are up to all day in the park. It would allow time to reschedule if things don't go as planned. If you hope to have a few hours in the park and then hop, or go back to the resort, it is a poor system.
We've decided that after many lovely vacations to Disney, we have had our fill. We won't be back for quite a while.