I never use the built-in AT&T/Cingular (the terminology varies) Music player (though I cannot uninstall it either, sadly). But it doesn't mean I don't use my phone for music. I have been exclusively using LCG Jukebox. Aside from being a generally good app, it does two things that are of note for all mobile apps.
First, its got a sort of widget. Instead of just making you go back to the app screen — or worse, stay there — the app runs in the background, and the widget tells you what is going on with the music player.
This is especially cool because nothing else does it. I didn't know you could add anything to that bit of the idle screen. Now, you can't get to it, so (as far as I can tell so far) you have to go to the app to pause or switch tracks, but presenting data live here is still really useful. I like it. Now, I want other running apps with useful data to share to do this also.
Second, the lyrics engine. You can search for lyrics, download them, and they they scroll back while the track plays.
This is not just useful and fun and entertaining, but a really neat way of using the network connection to bring text content into the device, join it with (probably sideloaded) media of a different type. This, not video, is what "multimedia" means to me. I hope to see more of it as app developers realize there is a speedy, always-on connection in our pockets.