Comment 51 for bug 199960

Uninstalling xserver-xgl is not a solution.
Without Xgl server (running on nVidia drivers only) Compiz is unstable, Emerald leaks memory as hell - this renders Compiz desktop unusable.
You may search launchpad for related bugreports. This is related to buggy off-card texture storage. Nvidia drivers are broken and push testures to system memory, but never frees it back. The more on-card memory you have, the longer it takes this bug to hit you, but it will hit you. So, you cannot count on a stable system with nVidia texture-from-pixmap implementation + Compiz.

So far, the solution was to install xserver-xgl. This gives very fast and stable Compiz desktop.
With this regression, one cannot have stable Compiz desktop on nVidia cards.

You will have many complaints by Gutsy->Hardy adopters if you do not fix this.

So far the overral Hardy experience for me was a major upgrade from havily broken Gutsy and Feisty.
This regression makes Hardy yet another Ubuntu release that breaks things that worked fine before...