I use TripIt to build itineraries for leisure travel, which is the only type of travel I do.
Usually, these international flights rarely have schedule changes so I don't need to know the gate assignment up to the minute.
I linked TripIt to my Google Calendar, which is set to automatically SMS me notifications so when I add events, I get SMS which is kind of annoying. Probably should turn that off. But it might be useful for calendar-obssessed types.
Also have PageNow Personal Finance.app and put in my Skymiles account details and it automatically loads my DL flights on there and sends push and email notifications. Again, I don't have to know these details up to the minute but it is nice to be able to easily store them on the iPhone without manually entering the data.
These are all free and have no desire to upgrade to the pay services. Maybe if I flew a lot for business, I would think differently.
Calendar.app on the iPhone is mainly a conduit for my Google and work calendars. I do have a Mobile Me account but I don't store the calendar up there. May not always have a Mobile Me account so I rather rely on Google Calendar or whatever is free.
That's the main value of the cloud services, that they're free. Otherwise, not worth the potential loss of privacy storing your data there.