Works nice! Also it's great, that during animation is seen wallpaper, helps to recognize which activity it is. Thanks!

This looks great, and it fills a major gap in KWin's repertoire. A couple small nitpicks if you don't mind:
1) Is it possible to add any configuration? It'd be nice to be able to set the duration of the effect (and even nicer if we could change the effect from a zoom to a simple crossfade or slide perhaps).
2) When switching activities, the wallpaper of the newly selected activity is briefly visible as the windows on that activity zoom in. That's fine, and I'm guessing it's the desired effect so as to serve as a quick reminder of which activity you're on, but it's not always consistent. A lot of times, the wallpaper of the activity I'm switching from will be shown instead. Is there any way to fix this?
Aside from those minor issues, this is pretty much perfect. Thanks, and keep it up!