> 1. They mention "Gnome compatibility" and "KDE compatibility" on their page. Why? I thought this window manager is supposed to be used instead of gnome/KDE, no?

You can do that if you want to, and most choose to. But there's good reason to not to:

I, for example, prefer not to spend a couple hours setting up dzen or xmobar in all their multifarious complexity & ability, getting the features right, setting up the colors to match my current GTK theme, etc. when I could just continue to use Gnome status-bar. (Gnome may have its issues, but I have no problem with the status bar.) Not to mention that running inside a Gnome session means you get a decent window manager, but also you get all the stuff Gnome usually does like printer support or device management. You could figure out how to do that independent of Gnome, but I don't consider that a good use of time.

> 2. On some screenshots you can clearly see overlapping windows, i.e. not all of them are always tiled?

You mean the floating layer? Windows are either tiled, or floating; floating is for things like Firefox dialogues which don't make sense to push into the tiled arrangement.

Gnome and KDE has their own window managers, namely Metacity and KWin. These can be replaced by other window managers, like Xmonad, and you can still use the Gnome/KDE infrastructure and utilities just as nicely.

For point 2, you can set a given window (or the layout for the current screen) to floating, which is the only behavior available in many conventional WMs. Some programs (most notably, the gimp) are very annoying to use in a tiled layout, so this is an easy way to accommodate them. .