I've found compiz to be horribly buggy and unstable to the point of not usable. And it doesn't like to play nice with KDE3's pager. But if it works for you, that's great. As far as window managers, I'm not even using one on my CarPC. It's all one app run directly on X, and the window manager will just get in the way.
i've heard compiz can be painful. I've never personally had a problem with it. I can't stand using a desktop without it.
If you only ever run one app, you are right, a window manager will get in the way. Another +1 for Linux: you can run without a window manager :P