TC - I do not want to open another bug, but I think that the scope of this one may be too focused on Tweak.  At initial upgrade to 10.10 from 10.04, noted Compiz could not be set as window decorator.  I was able to resolve by completely removing the package, as it showed it was being installed from Local in Synaptic.  

After last update on 10/10, my system defaults to Metacity, Compiz-Fusion icon will not start, settings made in CompizConfig Settings Manager do not "stick" (that is, they are back to default after I save and close and then reopen).  

When I do a compiz --replace:
d0ti5@Phideaux:~$ compiz --replace
Starting gtk-window-decorator
(gtk-window-decorator:6839): Gdk-WARNING **: GdkWindow 0x1e00153 unexpectedly destroyed
(gtk-window-decorator:6839): GLib-GObject-CRITICAL **: g_object_unref: assertion `G_IS_OBJECT (object)' failed
(gtk-window-decorator:6839): GLib-GObject-CRITICAL **: g_object_unref: assertion `G_IS_OBJECT (object)' failed
(gtk-window-decorator:6839): GLib-GObject-CRITICAL **: g_object_get_qdata: assertion `G_IS_OBJECT (object)' failed
(gtk-window-decorator:6839): GLib-GObject-WARNING **: invalid unclassed pointer in cast to `GtkWidget'
(gtk-window-decorator:6839): Gtk-CRITICAL **: IA__gtk_widget_get_window: assertion `GTK_IS_WIDGET (widget)' failed
(gtk-window-decorator:6839): GLib-GObject-WARNING **: invalid unclassed pointer in cast to `GtkWindow'
(gtk-window-decorator:6839): Gtk-CRITICAL **: IA__gtk_window_get_type_hint: assertion `GTK_IS_WINDOW (window)' failed
(gtk-window-decorator:6839): GLib-GObject-CRITICAL **: g_object_get_qdata: assertion `G_IS_OBJECT (object)' failed
(gtk-window-decorator:6839): GLib-GObject-WARNING **: invalid unclassed pointer in cast to `GtkWindow'
(gtk-window-decorator:6839): Gtk-CRITICAL **: IA__gtk_window_get_decorated: assertion `GTK_IS_WINDOW (window)' failed
(gtk-window-decorator:6839): GLib-GObject-WARNING **: invalid unclassed pointer in cast to `GtkWidget'
(gtk-window-decorator:6839): Gtk-CRITICAL **: IA__gtk_widget_get_toplevel: assertion `GTK_IS_WIDGET (widget)' failed
(gtk-window-decorator:6839): GLib-GObject-CRITICAL **: g_object_unref: assertion `G_IS_OBJECT (object)' failed

Then, I have no window borders or controls.  When I try to start the gtk window decorator (as per http://wiki.compiz.org/Decorators/GTKWindowDecorator), I can at least get my window borders back, but the command hangs forever.

d0ti5@Phideaux:~$ compiz --debug
compiz (core) - Debug: Could not stat() file /home/d0ti5/.compiz-1/plugins/libcore.so : No such file or directory
compiz (core) - Debug: Could not stat() file /usr/lib/compiz/libcore.so : No such file or directory
compiz (core) - Error: Screen 0 on display ":0.0" already has a window manager; try using the --replace option to replace the current window manager.

Launching fallback window manager

[hang]

I do get the same error in Ubuntu Tweak, clicking Compiz Settings:
Distribution: Ubuntu 10.10
Application: Ubuntu Tweak 0.5.7
Desktop: gnome

Traceback (most recent call last):
  File "/usr/lib/python2.6/dist-packages/ubuntutweak/mainwindow.py", line 437, in setup_notebook page = module()
  File "/usr/lib/python2.6/dist-packages/ubuntutweak/modules/compiz.py", line 229, in __init__self.create_interface()
  File "/usr/lib/python2.6/dist-packages/ubuntutweak/modules/compiz.py", line 247, in create_interface    
          hbox.pack_start(self.create_edge_setting(), True, False, 0)
  File "/usr/lib/python2.6/dist-packages/ubuntutweak/modules/compiz.py", line 362, in create_edge_setting combobox = 
          self.create_edge_combo_box("TopLeft")
  File "/usr/lib/python2.6/dist-packages/ubuntutweak/modules/compiz.py", line 339, in create_edge_combo_box setting = 
         plugin.Display[v]
  File "compizconfig.pyx", line 945, in compizconfig.Plugin.Display.__get__
  File "compizconfig.pyx", line 766, in compizconfig.Plugin.Update
KeyError: 'active_plugins'

What else can I give you?