I'm a frequent flier and the idea of a paper boarding pass seems so 1998 to me. My airline of choice (most of the time) is United Airlines. United has been doing a very good job with electronic boarding passes since it's inception several years ago. First it was an emailed link that would bring up a web page, then a email containing a PDF, app delivered boarding passes and now, integration with iOS Passbook.
When I first experienced Passbook in iOS 6 it was a very cool concept. I promptly loaded up all my rewards and membership cards that had been taking up space in my wallet. Did you know you can make a pass for a card or membership program that doesn't have an integrated service? When United updated their app for iOS 6 it brought with it the inclusion of Passbook boarding passes. At first this was revolutionary and the function of having a swipe to display boarding pass from my lock screen was amazing.
Over the past few months it seems that the passbook app has been going downhill, or at least the user experience. Lately I'm not getting consistent delivery of my boarding passes to the lock screen and very often when I swipe it brings up the wrong boarding pass (boarding pass for a future or past flight segment). I'm actually finding that the electronic boarding pass on the United app is a more streamlined experience than using the Passbook app.
As a related note I wish that Passbook offered an automatic delete feature. For example it automatically deletes the pass 72 hours after the pass expiration. I fly a lot, sometimes 6 boarding passes a week. At the end of the month it's not uncommon to have 30 or so older passes in passbook that need to be deleted.
I'm sure that Passbook is due for evolution, specifically as Apple enters the NFC and mobile payment space. My hope is that the app developers and apple will continue to develop this interface as the desired user experience is extremely beneficial to the consumer.