My upgrade to GCC 4.1 went perfectly well. But I think perhaps only because I've learned lessons in the past when things have gone terribly wrong. Such as, don't forget to recompile your kernel. And don't reboot before you emerge -e system.
Before I upgraded GCC, I tried to install Gnome 2.16. Horrible, horrible breakage. gnome-panel would no longer start. I tried to downgrade back to 2.14 and it broke even worse. control-center refused to compile at all, which means /usr/libexec/gnome-settings-daemon was missing, which means no GTK themes in KDE.
But since I knew I was going to have to emerge -e world after upgrading GCC anyways, I figured no better time to try something like that. Just remember that; any time there's a major system upgrade that will require a total recompile of all your packages, that's the time to start trying crazy experimental hard-masked alpha desktop environments. : )