A summary of my experience using MaxPass yesterday (Note: I didn't arrive until around 2:30, as my priority was Fantasmic not rides, so I haven't used it to its full potential yet):
I have Premier but the app still shows Signature. But when I got in the park, MaxPass worked automatically, so it was just an app glitch.
Loved:
-being able to book FPs once I entered the park. Typically when I arrive I'm immediately heading to pull my first FP. This let me have a leisurely stroll up Main Street stopping in stores and to watch Dapper Dans. By the time I made my way over to BTMR it was time to use (about a 20 min return time).
-Being able to cancel a FP. I initially got Indy as my second pass. Then I realized I should have gotten HM with and instant return and then Indy once I scanned in. It was easy to cancel the pass.
-getting access to a different pool of FP times. When I went to get a new Indy pass it actually offered a return time earlier than my previous pass! It conflicted with my dining reservation so I waited a bit. Every time I refreshed the app it offered different times all earlier than pulling a pass which was pretty cool!
-Switch to multi-experience FP. I eventually booked an Indy for after dinner but was so full I knew it wouldn't be a good idea so I didn't use it and forgot to cancel. But then when I checked the app I noticed it had switched to an any experience pass because the ride must have gone down briefly. And by brief, I mean maybe 10 minutes because every time I had checked the app it still showed it operational. I ended up not even using it because I was so tired after Fantasmic I just wanted to head home!
Annoying:
-It showed all my passes. So my DL ID, my WDW ID, and 3 Halloween party tickets. Every time I went to book a new pass I would need to unselect everyone and click just my DL ID. I also had to remember which one it was because it just showed the last 4 digits.
-App delays. When I scanned in at the first point for BTMR I immediately went into the app to try and book a second pass. But received the "Spinning Circle of Death". Even if I shut down the app entirely it still showed up when I reopened it and I was unable to get to the Redeem FP screen. Instead I used my physical pass to scan at the second point. So I'd absolutely recommend taking a screenshot of the redeem screen so you can use that in case the app messes up.
-Battery usage. Definitely sucked it up. But, I have a fuel rod so it didn't impact me much. That is something I'd absolutely recommend people get if using MP! You can get them online pre-trip for cheaper than at the parks. And you can swap it around to other people in your party so you only need one.