It is worth a look because the approach to use a modified window manager to enhance the Linux desktop is different to the one of Beryl/Compiz. I think most pressive is the ability to modify GUIs of applications at runtime. You can even save the modified GUIs to reuse them. So, if you want to have a button appearing somewhere else, well, you can just copy it to that place. That’s quite impressive.
Also the possibility to really resize windows on the fly by OpenGL instead of relying on the toolkit is impressive – with that technique you can fit the window size exactly to your needs. That is very, very helpful for people with eye problems.
Technically wise Metisse is an OpenGL enhanced plugin for the fvwm window manager (oh, and do I hate the ugly fvwm dialogs and menus in the videos!). It can be used with KDE as well is said in the FAQ.
In the meantime, Lubos Lunak was quite busy implementing more typical 3D effects like the ones of Beryl/Compiz into Kwin and has made videos available showing the effects and possibilities. Currently the videos are not available due to a Youtube misconfiguration, but I guess that will be changed soon.
Btw., looking at the title of this post I really see the need for a cool name of kwin_composite – any suggestions? Something like “cave” would be nice, because kwin is the place where konqi, a dragon, lives.