Matthias Clasen of Red Hat wrote to the GNOME developers' list and release team this week to make it known the GNOME 3.12 Wayland support will be classified as a preview. There's unfinished tasks and they don't want to merge the mutter-wayland branch back into Mutter until after the official 3.12 release.
GNOME 3.12 will work for basic GNOME Shell functionality as Wayland and an X11 compositor, display configuration, color management, message tray and hot corner support. Still being tackled for the 3.12 release target is support for launching Wayland sessions from GDM, using libinput for input handling, key bindings support, and Clutter apps support.
The items that don't look like they will make it for GNOME 3.12 with Wayland include input configuration support, Wacom tablets handling, start-up notifications, drag-n-drop and clipboard support, and status icons.
While it would have been nice to see official Wayland support for GNOME 3.12 that's being released in late March, now it will not likely be until GNOME 3.14 in September before it officializes their post-X11 support.
Our most recent benchmarks done atop a Wayland environment were from late March with Fedora 20's GNOME Shell on Wayland but some new OpenGL benchmarks and XWayland tests will come soon while using the latest Wayland, GNOME, and Kernel+Mesa driver code.
Clasen's GNOME 3.12 Wayland announcement can be read on the mailing list.