Wired has an interesting article/interview about iPods and the way that people tend to use their personal entertainment systems.
In terms of usage, Apple got it intuitively right. People use (the iPod) as an alarm clock, and when they listen to it at night, they like the fact it can turn itself off. It's how people like to use music. I don't think Apple did much research into how people would use their players, but they got most of it right.
For example, a lot of people use it to go to work, for commuting. I found that they use the same music on a regular basis. They will often play the same half-dozen tunes for three months, and each part of the journey has its own tune....
It gives them control of the journey, the timing of the journey and the space they are moving through. It's a generalization, but the main use (of the iPod) is control. People like to be in control. They are controlling their space, their time and their interaction ... and they're having a good time. That can't be understated — it gives them a lot of pleasure.
So, for example, music allows people to use their eyes when they're listening in public. I call it nonreciprocal looking. Listening to music lets you look at someone but don't look at them when they look back. The earplugs tell them you're otherwise engaged. It's a great urban strategy for controlling interaction.
It's also very cinematic. The music allows you to construct narratives about what's going on. Or you use it to control thoughts. A lot of people don't like to be alone with their thoughts. The best way to avoid that is to listen to music.
A lot of people don't like where they're going in the day. If you can delay thinking about that until the last minute.... People don't take off their earplugs until the very last minute, until they're inside the door at work. It's a great way to control mood and equilibrium.
Something to remember both when writing music and designing technologies to support music. Give the people what they want, make it very user friendly, take them a bit away from the normal day-to-day and you'll find success (or, more accurately, success will find you).