We had a disappointing experience in several ways. First, we went through 2 sets of MagicBands. Of the first set, one worked to open the WDW hotel room door and to charge, the other never worked. Neither would work as our park tickets, and therefore no FastPass+. On our second day we got a new set of MagicBands. They worked even less. No longer did they even open our hotel room door. Only one worked for charging. This new set ALSO didn't properly tie into our park tickets, despite multiple tries over and over again, and therefore once again no FastPass+. Guest services were very friendly and tried to help, but never were able to resolve the issues that we had with TWO separate sets of MagicBands. I wasted hours at the Coronado Springs guest services, and at GS at EPCOT and MK. It was a severe waste of time, money, and effort - on what is supposed to be a vacation where I can relax, be spontaneous, etc, etc, etc.
Therefore, I can safely say that I am disappointed with the system. $1.5B gave us armbands that don't do anything that our park tickets and Key to the World cards didn't do before...and couldn't do now. FastPass+ might be a good idea in concept, but in practice, it failed us. Additionally, with no old-school FP available in the parks for multiple days when we were there due to the FP+ demand, it really hurt our image of the resort. Gone are the days of relaxing and experiencing attractions. Now I need to book my ride times in advance or I don't get to have a pleasant experience.
WDW has one simple goal with MyMagicPlus and those MagicBands: to get me to spend more money. Not to increase the number of attractions, or upgrade attractions, or draw me into the parks with better entertainment. It is just to make the flow of money easier. Well, they failed, because their equipment didn't even work. So, we spent less money, and we aren't planning a return trip to the "Pre-Booked Attraction, Time Share Kingdom" for next year.
Good luck on your trip. Have fun. I hope you have better luck than we did.