I have Ubuntu 9.04 installed under VirtualBox 3.0.2 running in Windows 7.
With Compiz enabled, all animations are performed smoothly and without flaw save for this one notable exception - the final frames of any animation are not updated to the screen. Only when the screen is again updated does the previous animation get drawn.
Unfortunately, this is not limited to fancy animations but applies to any screen update. When typing, a character does not appear after a key-press until the following key-press, so text boxes always appear to have a one-character lag.
However, if there is a continual on-screen animation (such as a progress bar), all screen updates seem to complete.
I've attached two screen-shots. The first is after closing a window, and the second is after clicking on a menu. The desktop would remain as shown in the screen-shots until something else was updated on the screen.