I had a compact flash-based RCA Lyra before my iPod, and it wasn't so much swapping out one chunk of music for another that bothered me, but the necessity of determining what I'd want to listen to in advance. Just as soon as I'd come up with the perfect playlist, my mood would change, or I'd get sick of one of the songs in it, and either I'd have to re-load the player or shorten my listening time skipping what I no longer felt like hearing.
The iPod, by contrast, can carry enough music to switch between moods, artists, genres, playlists, or what-have-you, on the spur of the moment. And the Shuffle-Songs feature lets the iPod play the DJ. It really is like your own personal radio station, 24-7; or even your own personal soundtrack.
I've got a playlist for New music that I'm still getting acquainted with, but if I get tired of that and yearn for the familiar I can switch to the Good in 2003 list that contains everything with a rating of three stars or better that I've put into iTunes this year. Or I can bring back blasts from the past with the non-chronological Three Stars playlist. Or indulge a sudden genre craving with the Electronic playlist. It's all there, all the time.
Even with just 1,111 songs loaded to my iPod, the freedom to change up at will like that has changed both how I listen to music, and the rate at which I acquire new music. I've bought more music this year, both at the iTMS and as tradtional CDs, than I spent on music for the three previous years total!