It's been a while since Mir was announced. Ubuntu Mir is still under heavy development, of course, but it's expected to be "stable and usable" in time for Ubuntu 13.10 since it'll be included with it. If experience with Unity is any indication, we can expect Ubuntu 13.10 to have quite a lot of issues on release. I guess we'll know if history repeats itself soon-ish.
So what's been cooking for Mir? Lots, but here's some of the more interesting bits.
DPMS support was added for a short while before getting reverted due to problems in revision 1062. Bug #1193222 is still not fixed as a result.
Composite bypass support bring a nice performance bump to full screen apps (games mainly). This only effects full screen apps, however, so doesn't affect the regular desktop. That will have to wait for 2D performance improvements.
Finally, Intel has dropped support for Mir in their GPU drivers. Ubuntu will have to maintain their patches for the Intel drivers on their own from now on.
If DPMS support doesn't make a comeback by the time Ubuntu 13.10 is out then there's one regression for you already since that's working in current Ubuntu. We'll just have to wait and see.