Used passbook on my current trip. A few thoughts:

I normally check-in online using a laptop or browser on a tablet so having to check-in using the UA mobile app and then sending the boarding pass to passbook felt like an extra step. Especially since I could just get it through the mobile app.

I didn't like that the flight shows on the locked iphone screen. It was there 6-8 hours before the flight and just made me feel like I was constantly going to be late for my flight. You can click the info button in the bottom right of the screen to turn this feature off.

Supposedly having it show on the locked screen will alert you to delays etc. When I landed at NRT I did get a pop-up saying the gate had been changed (really assigned) but that was a nice feature. But I also got an email from United saying the flight was departing late and the boarding pass in passbook still shows the original time. Would prefer to have it update with the new departure time.

I don't have anything else in passbook so the only thing showing are my two boarding passes for UA. If you had several things in passbook it might be annoying if you have to scroll through them all to get to the boarding passes.