My son and I have spent many nights out in the mountains with Star Walk fired up. We would scan the night sky identifying constellations and looking up information in their Wikipedia entries.
Vito Technology just recently updated the iPhone version to 4.0, most likely so that it and the iPad version would share all the same functionality. Obviously, everything is going to look better on the iPad's larger screen but it's good to know that they are keeping parity between the apps. And remember, Star Walk was featured in Apple's Best Of 2009 list.
Both include a digital compass, which traces the sky as you move about; constellation images superimposed over stars; and a time slider that lets you scroll back and forth through time to see what the sky looked like at different points in history or in the future.
Star Walk allows you to bookmark your favorite observations of the sky. It gives you info on the phase of the moon as well as rising and setting times of various solar objects such as Jupiter, Mercury, Venus, etc.
And this is just the bare bones of the app. Star Walk is an incredibly feature rich astronomy app. Definitely in the running for Best Of 2010.