I actually had a play with an iPhone today and the screen wasn't too bad to be honest. Seemed similar to the 770/N800 in terms of what one could see (I understand the dpi and resolution is lower). The UI was very snazzy, but it was a bit slow.

I use rockbox on my iriver, works well (though looks a bit utilitarian) as it's able to do on-the-fly playlists unlike the standard firmware.