Aero does impair gaming performance as it disables itself on fullscreen. You can see this for yourself when you compare fullscreen to windowed. Case closed.
Aero is indeed very stable, but Linux is in constant development and deployement. Maybe Aero was considdered stable before Vista for all you know.
But... Windows has come a long way in terms of stability! Now they must focus, and they are with Windows 7, on speed and security improvement. With the legacy cruft and tools from the stonage it is inarguable that the people who coded key parts of Windows 7 are gods.
That doesn't take away that Linux is more awesome alround, though.
My experiences with the recent (Late 2009 / early 2010) Catalyst releases, on a HD5970, agree with Michael's tests. In fact, I often find that everything -- 2D and 3D rendering -- is much, much faster with Compiz enabled than otherwise!
For instance, with Chrome (which does 2D rendering for normal text and such), scrolling the screen without compiz provides a very noticeable tearing effect and a "long" delay of about 0.25 - 0.5 seconds before the screen refreshes to display the new content. With compiz enabled, scrolling is as smooth in Chrome on Ubuntu as it is on a Mac Pro in Safari -- and we all know Apple has OCD when it comes to smooth scrolling.
I do indeed praise ATI for finally providing an optimized composited desktop. They still have some bugs to work out in the 3d driver, but it seems fglrx isn't the bogeyman it used to be. And yeah, they need to better keep up with Xorg and kernel releases
Still notice bad tearing when dragging windows on the desktop, but only when moving the window fast.