I like having one device for all my stuff, as opposed to the old days when I lugged around a flip phone, Palm, and iPod, but the locking iPhone screen creates issues when I'm using it as an iPod. Notably, it's a big pain in the ass to move around in a song list, and if you want to see what track is playing it comes up in a tiny display unless you bring the whole iTunes app to the front. None of which anyone should be doing while driving, which is what I'm often doing when I'm listening to music.
The other day I happened across a $1 app, FlickTunes, which has increased my musical quality of life considerably. It gives the iPhone touch controls that involve the entire screen, so you don't have to peer at it, and it displays a giant font that reads what's playing. You can tell it not to autolock while the app is active, so while I'm driving, for example, I have it on a charger anyway and just leave the screen on. Swipe left or right for next/previous track, up to play/pause. Use a two-finger swipe to increase/decrease volume. I'm loving it for workouts as well; if the iPhone is strapped to my arm, without FlickTunes I have to unstrap it to look at the screen to make any changes. Big PITA. My point is that this app is crazy useful and definitely worth the $1.