I've been testing the newly released Fedora 22 with MATE 1.10.0 and it looks pretty good, so far.

The big change is the inclusion of DNF (Dandified Yum) package manager, as a replacement for yum. The UI for DNF is more polished than the UI for yumex. However, there are still a few minor issues to work out. Once the few release bugs are ironed out, it should be a nice package manager. If you prefer, yum can still be used, but this is Fedora, so move on or get left behind.

MATE 1.10.0 is now included and has a little better support for GTK3. You can read about some of the changes here. It seems stable, although I did have a panel app freeze up on me once. That was just a minor annoyance, though.

I understand that MATE DE has actually been fully ported to GTK3. The developers are reluctant to "officially" release it. It seems they don't have the resources to keep up with all the ongoing API changes to GTK3, while still being able to focus energy on the MATE DE itself. The thought is that perhaps at some point in the (not so distant?) future, we may see GTK4. When that happens, development on GTK3 will slow, allowing them to release a GTK3 version of MATE that is more manageable. I'd imagine the same thought has crossed the mind of the GIMP developers, as well.

I haven't tested any other Desktop Environments but I understand Gnome3 is stable. KDE has moved from V4.0 to V5.0 and there are some issues to be resolved there. No reports on any major issues with LXDE or XFCE.

You can download the latest Live ISO images for all the popular DE's, as well as specialty spins via Fedora Labs, from the following link: http://spins.fedoraproject.org/