Delta's is pretty good on the iPhone. Their iPad app is S-L-O-O-O-W and pushes deals too much.
I use the iPhone app for schedules, tracking bags (it's not 100% at that) and check in when traveling domestic. I only just added my passport info so I won't know until Monday if it allows international travel.
I wish that it would use fingerprint login though, and that it wouldn't automatically log you off if you started it up with no internet connection available.