The BB works great as an MP3 player, but if you're listening to it and you get a message/call/sms, etc it will stop your song to play the notification. I'm sure if you put it on silent/vibrate it would be fine.
There's a great app called FlipSide for Blackberry and it's a great mp3 player program. If you're planning to use your blackberry as an mp3 player, I recommend buying it.