The Gnome Shell graphical performance is a issue for me. I have 3360x1080 pixel desktop with Intel drivers and when I do 3D games or 1080i/1080p video playback I need all the muscle I can get.
Compiz itself has improved dramatically in the last couple years. It's fast and stable. I can't really tell the difference in performance between Compiz vs plain-jane Metacity. I have not tried Gnome-Shell in a few months, but it got to the point were using it was fairly painful because of the slowness. For normal desktop stuff it was great and I prefer the shell layout, but it's just slow.
The way I figured it, though, was that Mutter would improve and the drivers will improve.
This Unity shell is really very ballsy and ambitious move for Ubuntu. Good luck to them as if they can pull it off it could mean big things for them.
On a side note:
It looks like Gallium drivers are really coming together now. I am really thinking hard about getting a new AMD desktop with ATI graphics.... Fantastic work on the OSS drivers. Really cool stuff.