Currently when you move window from one desktop to the other with Slide effect enabled it looks like this: http://www.youtube.com/watch?v=iDHsHoOiPqs

That is, when you move window to another workspace it disappears, and then appears at the new desktop, and it doesn't look intuitive, you don't really have a feeling that this window is moving to another desktop. Animation was slowed down for the above video, in reality you don't really understand what happens (try playing that video on 2x speed, but this won't give you the real speed).

In compiz however, window that is being moved is fixed relative to the viewport. So that you really see the animation how this window moves to another desktop. It looks like this: http://www.youtube.com/watch?v=gVTppLvXnUc#t=271 This one is much more intuitive - you see your workspaces switching and window moving from one workspace to the other.