My Gnome Shell is working just fine - for me
I am not a gamer. My comment is more for a normal desktop user.
I installed Kubuntu, then added the Gnome3 PPA. I added just the Gnome3 stuff and avoided all the legacy stuff from a standard Ubuntu install. I reconfigured the login to GDM and now have a stable Gnome Shell desktop - with KDE in reserve.
I had to remove the fglrx drivers as they were unusable. The screen was corrupted constantly. I installed a 2.6.39 kernel and xorg-edgers and am now running Mesa/Gallium3D.
All animations are smooth. Xvid videos play full screen smoothly. Flash via YouTube plays smoothly. I have WebGL running in Chrome and the demos run smoothly. I had to use the --disable-gpu-blacklist flag in Chrome.
My family have all had issues with Unity. One of the worst is the launcher being stuck on the screen and overlaying running apps. With Gnome Shell there has not been any issues of that nature. I need KDE installed as well. The new Nautilus cannot handle Samba shares, but by using Dolphin under Gnome it all works nicely. All the KDE networking stuff is installed and usable. My media players happily read my shared folders.
So, while all the above benchmarks point to gaming being near to impossible under Gnome Shell, I have no problems with web browsing, video, word processing etc. And my desktop and desktop effects are rock solid.
And I do agree with an earlier post that suggested trying the benchmarks with Fedora 15. It will be interesting to see what happens in a dedicated Gnome Shell distro. Using the Gnome3 PPA under Ubuntu is a recipe for dependency hell. Using it with Kubuntu is, so far, a better option for me.