I use Maximus to undecorate windows, using the top GNOME panel to house the Window buttons and Globalmenu apps. The Metacity "compositing manager" option must be turned off, otherwise every panel disappears each time I close a window. Any help appreciated.
Install xcompmgr and add it to your startup applications.
Xcompmgr is a compositing manager that works in most situations where other compositing managers don't, and can even be used alongside window managers such as openbox and icewm, or metacity (with compositing disabled in metacity).
Are you using the window buttons from the following PPA: ppa:tsbarnes-misc
If so, there is an option to hide Compiz decorations for maximised windows in the preferences (right click on the window buttons and select preferences). It is on the bottom of the "Behaviour" tab.