XMir's been delayed from Ubuntu 13.10. The stated reason is that multi-monitor support isn't sufficiently reliable. That's true, but it's far from the only problem that XMir still has:
* It's still broken on some single-monitor systems
Some machines have display corruption. There are writes to freed memory. To be fair, some of the behaviour that's been seen has been down to underlying bugs in the Xorg drivers that were never triggered under normal use but are hit by XMir. Others are down to implicit assumptions made in the drivers that XMir happens to violate. The problem is that there doesn't appear to have been enough room in the schedule to deal with these interactions, presumably because nobody accounted for the inevitable "This thing we thought would be easy turns out to be difficult" part of the project.
* The input driver bug still isn't really fixed
rest here