Although the new Unity experience is a smooth and snappy one desktop wise, video playback is a little choppy and sluggish. Even when using GPU powered VDPAU compiz is using a lot more CPU than it should and playback all in all is not enjoyable for me. Unlike earlier releases tearing is not a problem though.
Installing Gnome Shell works much better for video. But the new 3.4 desktop feels not as good as 3.2 did. So not a real option either.
Fortunately there is xbmc with its great media center software. For the first time available from standard Ubuntu repositories. Using it from within either Unity or Gnome suffers from the underlying desktop environments. Choosing the xbmc session from lightdm however delivers what any media enthusiast would want. Playback is smooth, CPU is very low and I’m happy again.