It was posted today that delaying Mir + XMir + Unity 7 on the desktop for supported graphics cards in the Ubuntu 13.10 time-frame won't happen due to "outstanding technical difficulties", but they are at least in shape for delivering a proper Mir experience on Ubuntu Touch (mobile) platforms.
The issues publicly pointed out aren't about Mir itself but rather the XMir portion that provides the X11 compatibility layer for applications to run on the Ubuntu display server. Unity 8 will be Mir-native but Unity 7 as will be shipped in Ubuntu 13.10 is not. One of the biggest XMir issues right now is improper multi-monitor XMir support.
Initially there were also some big Mir/XMir performance problems, but fortunately those issues have been worked out in recent weeks.
The proprietary AMD and NVIDIA graphics drivers also don't yet support Mir, but Canonical had already planned to use an X.Org Server fall-back for Ubuntu 13.10 and they claim that the GPU vendor support of Mir is "progressing forward."
For those that want to test Mir in Ubuntu 13.10, it will still be available in the archive as a non-default setting. Additional details on this last minute change for Ubuntu 13.10 can be found via fridge.ubuntu.com.
Canonical hasn't yet said whether this delay in the default adoption of Mir/XMir will affect their plans to ship Ubuntu 14.04 LTS by default with Mir, but it's a possibility now at this point. We'll, of course, continue monitoring the Mir situation at Phoronix as we have been objectively doing from day one.