Here are some things I learned this trip about FP+. Caveat, they kept telling us that this is still a trial and that things may change.
First, I HATE it, HATE HATE it. Really We were masters of the old system and almost never waited in a line longer than 10-15 minutes. We park hop a lot and liked to get FP wherever we were. The current system limits you to 3 FastPasses in one day and only at one park. Previously, you could also get FPs for people in your party and use them for others. My MIL and FIL were with us this trip. In the past, older DD and DH could have used their FPs to ride a ride again. But now that isn't possible because the FP are linked to each ticket/Magic Band.
One thing I did like was that you could change your FP on the fly in the park with the MDE app. So for example, we realized we weren't going to make a FP for Space Mountain. So I switched it to a slightly later time. You can also do it if you have missed the FP window. Of course, this only works if FP times are still available.
What we ended up doing a lot was going to the ride/show/greeting, and if the current Standby wait time was acceptable to us, we just waited in line and switched for a different FP somewhere else at a later time.
Also, once you have used a FP in a park, you can't switch any other FP to another park. Since we park hop, this was the worst part for us. So one day we went to AK in the morning and planned to go to MK at night. What we did was have FP set up for 3 rides in AK for the morning. When we were able to go on 2 of them without using a FP, I went ahead and switched our 3 AK FPs for FPs at the MK later in the evening. That was a VERY wise move on our part because the MK was very crowded that night (it was a Saturday night).
Currently, the Magic Bands make park entry excruciatingly slow. Many people really struggle getting the bands to read (holding the band at the wrong angle, etc.). Also make SURE your kids do NOT know your PIN number if you don't want them to charge! My ILs actually checked into the hotel, so maybe this can be changed. But younger DD loved to "buy" things in stores with her band. If she had known the PIN (and was old enough to be on her own) she could have easily gone to town charging LOL!!!