XBMC 12.3 provides many bug-fixes that were back-ported from the current XBMC 13.0 development work. The 12.3 update has OS X 10.9 fixes, PVR add-on updates, PVR bug-fixes, memory leak fixes, audio channel mapping, possible crash fixes, and much more.
The next major update to XBMC will be version 13 and it will premiere early in 2014. Future Phoronix articles will cover the XBMC 13 exciting features.
More details on XBMC 12.3 can be found via XBMC.org.