Most of the airlines I use mainly in Northern Europe support passbook (AY, AB, BA, etc and AA in US), I use it often.

With the Apple Watch passbook I have gotten the "wow that's so cool" response around here at the airport when I've tried it (Apple watch is not on sale or commonly seen in most European countries yet).

Scanner placement usually makes it more convenient to use the phone as you need to hand your boarding pass to the staff for scanning at their counter. I hope this may change soon when more users appear.

When the scanner placement is OK for using the watch, it is really cool and easy, though.

Sometimes I also print the boarding pass, as the passbook version usually doesn't show clearly enough priority boarding, lounge invitation etc.