I think the stock music player is okay. But my biggest gripe is the delay or lag between songs. This may not be apparent if each song has its own track, thus no problem. However, if one song is on multiple tracks, then you will hear a break in the music when it switches from track to track. This is not a problem for most, but for one of my college classes (Music) it becomes annoying. They take a song and break it down to different tracks to split up the elements of the musical piece. On my 3GS, the music plays flawlessly, never noticing the switch between tracks. So its all seamless like it is on one track. However, on my Incredible, you have a slight pause when it switches tracks. So if you are listening to Beethoven's Symphony No 5, you'll hear the music pause for a split second every 30 or so seconds (or however long the tracks are). Annoying.
I even tried putting the music directly on the phone vs. my micro sd card. Same issue. I wonder if other music players are better? I'll have to look around.
Anyways, this will not be a problem for most people. It's not quite as fast as the iphone 3GS when loading up huge music or song lists. But once its in memory, its plenty fast. Its decent.