I've been playing around with Clutter lately and find it intresting project. So shortly it's a library to create fast, visually rich and animated graphical user interfaces. It uses OpenGL or OpenGL ES for rendering, but hides their complex API.
It uses GObject as base of it's classes and people who has been coding with GTK or glib are very home with Clutter. Additional libraries provides features like GStreamer playback support, Cairo rendering and embedding into GTK.
Building an simple application seems to be easy following the documentation. There seems to be lot of features, and basic operations works nicely. I made a test app utilizing clutter-gtk for GTK embedding, an it run smoothly on my desktop. However when I tried same application using OpenGL ES backend I faced problems. Clutter was missing XEMBED support for ES backed. I found this support in normal OpenGL version and find out making support for ES was quite easy job. I made a patch and posted it into Clutter's Bugzilla. Now I got a response and they seems to be excited about this patch.
It seems that Clutter is a nice project and the project team members are doign a good work. The library could be used more widely to make some nice, smooth UI's. However I'm not a graphic so didn't manage to create too smooth UI, but maybe somebody else success more better than me.