I use flipside on my 8900 and like it much better than the default player.
Flipside is cool because it scrobbles the songs to last.fm. I wish last.fm would get a Blackberry app, so I could listen to my friends' and neighbours' last.fm libraries on my Blackberry. I'd give up Slacker in favour of last.fm if that feature was available.
Posted from my CrackBerry at wapforums.crackberry.com