My biggest complaint with the iPhone is it's battery life. After setting up the iPhone to sync both my gmail and exchange mail, I was lucky if the battery would last until I got home from work.
Turning off the Wi-Fi helped a lot. Over the weekend I turned off the auto syncing of mail and all of a sudden, the battery seemed to last forever.
Thus far, I have found two "killer apps" that are really making me love the iPhone.
The first is Shazam. Ever heard a song in the background and wonder, what song that is? Well, you don't have to wonder any more. Shazam has the ability to pick out songs in loud places and determine what is playing. It then provides links to iTunes and YouTube for the song.
The second apps is Pandora Radio. You tell Pandora a song you like, then it starts streaming songs similar to it over the iPhone. A few votes up or down on what it selects, then you have a stream full of songs you like. Now I have the ability to stream internet music in my car. Sweet! Best part, it doesn't seem to kill the battery (that fast).