I have a Lumia 521 with the Dev. Preview. When I hit the volume rocker, while playing music, I get to see which song I'm on. Then when I hit the forward button, to switch to the next song, it lags, and I never get to see what the next song is. Windows Phone 8 didn't have such a lag, and I could switch the songs almost instantly. Small Gripe.
MS already announced that there will be an update to xbox music soon so hopefully that will take care of your issues.